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Light is essential for primary production and, therefore, its attenuation controls the vertical distribution of plants and phytoplankton over the water column. The diffuse attenuation of irradiance (K) is mainly governed by the attenuation by the water itself and the concentrations of optically active substances (e.g., phytoplankton, inorganic particles and colored dissolved molecules), which makes it an important parameter for eutrophication monitoring. Over the past century, Denmark has had recurrent eutrophication events, with extreme episodic cases where anoxic conditions were observed. Since the 1980’s, eutrophication in Danish waters has been monitored with regards to the diffuse attenuation coefficient of scalar irradiance (Ko) of photosynthetically active radiation (PAR, 400–700 nm), Ko(PAR). However, radiometric measurements in Denmark are difficult in winter due to low solar zenith angle and only few light hours. On top of that, radiometric measurements in the first meters of the water column are highly affected by light refraction influenced by waves, compromising the monitoring of shallow turbid waters as in Denmark. Therefore, we developed a semi-analytical model based on data to from a spectral AC instrument (AC-S, Sea-Bird Scientific) that can estimate the underwater light field and the diffuse attenuation coefficient of downwelling irradiance, Kd(PAR). We tested two distinct approaches based on equations from the literature for estimation of Kd(PAR). The results show that modeled PAR profiles follow the overall shape of in situ radiometric profiles but with smoother profiles, especially in the surface layer (2–5 m). Along with that, the method provided robust Kd(PAR) estimates, that were strongly correlated to the reference Ko(PAR) values from in situ profiles and with low root mean square error (RMSE). Thus, AC-S data can be used to estimate the underwater light field and Ko(PAR). This will make possible to retrieve Ko(PAR) in the absence of daylight and, therefore, allow for environmental monitoring outside the daylight hours, making environmental monitoring more efficient. In addition, the method provides valuable insights into the factors controlling light attenuation.

The study is based on a very extensive data-set of physical, biological, and optical parameters from below the sea ice in the western Amundsen Basin, central Arctic Ocean, in August–September 2012 during the record low sea ice extent. The water column was strongly stratified at all stations related to salinity differences between a surface layer of reduced salinities (34). A nitrate utilization-based budget in the surface layer gave a primary production of 67.5 mg C m− 2 d− 1, which reduced to 3.9 mg C m− 2 d− 1 in August 2012. Amundsen Basin primary production rates are lower than rates determined for other Arctic Ocean deep-water basins, and also lower compared to rates on the shelf. Below ice phytoplankton was well adapted to low light conditions in the Amundsen Basin and the photosynthetic potential was high, but limited by the low nutrient fluxes induced by the strong stratification. Amundsen Basin is foreseen to be ice-free in summer in 3–4 decades, and the question whether primary production will increase when ice-free was resolved with a coupled physical-biogeochemical model. Results showed that production will increase 10 to 14 times from the present 3.9 mg C m− 2 d− 1 to 37.4 and 55.2 mg C m− 2 d− 1 for an ice-free August and July–August, respectively. The study substantiates that both present and future ice-free low production rates were related to the strong stratification, reduced nutrient fluxes, and deep lying nutrient rich waters. Low production rates and strong stratification are discussed in the view of parameters that increase this stratification as higher freshwater run off or reduce stratification as wind.

The effects of previous photosynthetic activity on dark respiration rates were investigated in Heterocapsa rotundata (Dinophyceae), Rhodomonas salina (Cryptophyceae) and Thalassiosira weissflogii (Coscinodiscophyceae). Dark respiration rates were measured after 1 to 3 h of light exposure in 12 bottles where incubation irradiances ranging from 0 to 600 μmol photons m − 2 s − 1 . The results showed that the dark respiration increases with increasing light exposure (both irradiance and duration) in all exponentially growing cultures but not in two experiments with cultures in stationary phase ( T. weissflogii ). This relationship could be described by a linear model for all species, relating respiration to the previous accumulated photosynthetic product with a slope of about 0.035 h − 1 , indicating a growth-related respiration of 3.5% per hour of the newly fixed carbon. The maximum respiration rate (R max ) was estimated from both light dose and accumulated photosynthetic product using a non-linear model. Although R max and light-saturated gross photosynthesis (P g max ) varied between the three species, a similar R max : P g max ratio of ~ 0.12 was found. The maintenance respiration rates which are comparable with maintenance respiration rates from literature, varied from 0.004 to 0.02 mol O 2 g − 1 Chl h − 1 for the three species. Overall, this study suggests a diel periodicity of phytoplankton respiration driven by accumulation of newly fixed carbon in light. The consequence is a decreasing respiration rate during the night or during long dark incubations.

Carbon-to-chlorophyll a ratios (C:Chl a; weight : weight) were analyzed for 7578 coastal seawater samples collected from Danish waters from 1990 to 2014. The aim was to identify the seasonal and spatial dynamics relative to nutrient richness and to study the effect of reduced nitrogen loadings over time. C:Chl a values were lowest during winter, about 15 across all stations. During the spring, C:Chl a increased to summer values between 20 and 96, depending on the annual mean of total nitrogen concentration. An inter-annual sinusoidal model with monthly time steps described the seasonal C:Chl a pattern well. The amplitudes of the model varied inversely with the annual mean of total nitrogen. Data also showed that a reduction in nitrogen loadings to the area by ∼ 40% over the past 24 yr, resulted in a statistically significant increase in mean annual C:Chl a values of 0.8 ± 0.2 yr−1. The patterns derived from this large data set can be used to predict C:Chl a values for temperate coastal phytoplankton. Use of the empirical relationships derived from the data set improves predictions of C:Chl a values and thereby, e.g., carbon based food-web calculations and carbon-based ecosystem models, which often are validated using chlorophyll measurements.

We used continuous measurements of dissolved oxygen (DO) in dark bottles to characterise patterns of the dark respiration rate (R dark) for three marine phytoplankton monocultures and in natural-water samples from two marine coastal systems. Furthermore, patterns of ecosystem community respiration rate were determined from open-water changes in DO in a fjord and in a lake. We considered two models of R dark to describe temporal changes in DO: Constant R dark and decreasing R dark; increasing R dark. In addition, the effect of incubation time on R dark was investigated in bottle incubations. Constant R dark was observed in short-term (12-h) bottle incubations in natural-water samples from two marine coastal systems. Declining R dark was observed in marine phytoplankton cultures and open-water measurements in a lake. Increasing R dark was observed in open-water measurements in a fjord, particularly during summer. Long-term (120-h) bottle incubations in natural-water samples showed an increase in R dark after 48 and 72 h. We show that the conventional expectation of constant rates of respiration in darkness is far from typical, because non-linear changes are common under both controlled experimental conditions, as well as for open-water measurements of ecosystem respiration.

Monitoring data (1999 to 2012) and data from a 2 wk field study at a seasonally strat- ified station in the Aarhus Bight near the Danish coast were used to demonstrate that the vertical distribution of photosynthesis influences both water column oxygen conditions and the fate of the organic material produced. The primary production (PP) occurring below the surface layer, i.e. in the pycnocline-bottom layer (PBL), is shown to contribute significantly to total PP. Oxygen concen- trations in the PBL are shown to correlate significantly with the deep primary production (DPP) as well as with salinity, phosphate loading, wind and transparency in the surface layer. The phyto- plankton communities detected in the surface layer and PBL during the field study were very dif- ferent. Large cells, especially Ceratium spp., dominated in the PBL, while small diatoms, mainly Proboscis alata, dominated in surface waters. On the basis of chlorophyll-normalised photo - synthetic parameters and variable fluorescence, it is shown that the 2 populations were physio - logically distinct. The population in the PBL was photosynthetically active and adapted/ acclimated to lower light than the population in the surface layer. Sinking rates (based on sedi- ment trap collections) of carbon and nitrogen were highest in the PBL. Lyngsgaard et al. (2014; Limnol Oceanogr 59:1679�1690) have demonstrated that the vertical distribution of PP in this region is influenced by anthropogenic nutrient loading. Thus, the present study indicates that eutrophication effects may include changes in the structure of planktonic food webs and element cycling in the water column, both brought about through an altered vertical distribution of PP.

The seasonal variation in phytoplankton activity is determined by analysing 1385 primary production (PP) profiles, chlorophyll a (Chl) concentration profiles and phytoplankton carbon biomass concentrations (C) from the period 1998-2012. The data was collected at six different stations in the Baltic Sea transition zone (BSTZ) which is a location with strong seasonal production patterns with light as the key parameter controlling this productivity. We show that the use of Chl as a proxy for phytoplankton activity strongly overestimates the contribution from the spring production to annual pelagic carbon flow. Spring (February and March) Chl comprised 16-30% of the total annual Chl produced, whereas spring C was much lower (8-23%) compared to the annual C. Spring PP accounted for 10-18% of the total annual PP, while the July-August production contributed 26-33%, i.e. within the time frame when zooplankton biomass and grazing pressure are highest. That is, Chl failed in this study to reflect the importance of the high summer PP. A better proxy for biomass may be C, which correlated well with the seasonal pattern of PP (Pearson correlation, p

The land-to-ocean flux of organic carbon is increasing in glacierized regions in response to increasing temperatures in the Arctic (Hood et al., 2015). In order to understand the response of the coastal ecosystem metabolism to the organic carbon input it is essential to determine the bioavailability of the different carbon sources in the system. We quantified the bacterial turnover of organic carbon in a high Arctic fjord system (Young Sound, NE Greenland) during the ice-free period (July-October 2014) and assessed the quality and quantity of the 3 major organic carbon sources; (1) local phytoplankton production (2) runoff from land-terminating glaciers and a lowland river and (3) inflow from the ocean shelf. We found that despite relatively low concentrations of DOC in the rivers, the bioavailability of the river-DOC was significantly higher than in the fjord, and characterized by high cell-specific bacterial production and low C:N ratios. In contrast, the DOC source entering via inflow of coastal shelf waters had high DOC concentrations with high C:N and low specific bacterial production. The phytoplankton production in the fjord could not sustain the bacterial carbon demand, but was still the major source of organic carbon for bacterial growth. We assessed the bacterial community composition and found that communities were specific for the different water types i.e., the bacterial community of the coastal inflow water could be traced mainly in the subsurface water, while the glacial river community strongly dominated the surface water in the fjord., This study was funded by research grants from the Danish Ministry of the Environment (DANCEA), the project MicroPolar (RCN 225956) funded by the Norwegian Research Council, Carlsberg Foundation and the Arctic Research Centre at Aarhus University. CS was funded by the Danish Research Council for Independent Research (DFF 1323–00336). The Arctic Science Partnership and the Greenland Ecosystem Monitoring program facilitated this work.

The study is based on CTD and water sampling in the Eurasian Basin of the Arctic Ocean in August–September 2012. The research icebreaker Oden and a helicopter were platforms for the sampling. We here report data for inorganic and organic bound nutrient (NO3 −, NO2 −, NH4 +, PO4 3−, Si(OH)4, DON, DOP) concentrations and chlorophyll a between 10 m and 200 m at 29 stations and down to about 4500 m at 13 stations. The data set also includes below sea ice samples (1-2 m) sampling at 31 stations. A total number of 15 different water masses were identified, and their N:P relationship showed that 13 of these clearly originated from the Atlantic Ocean.

Anthropogenic nitrogen (N)-loading has decreased significantly in the Baltic Sea Transition Zone over the past two decades. We show that the vertical distribution of primary production (PP) changed as a function of landbased N-loading using 1385 water column photosynthesis estimates, in which photosynthetic parameters were determined both in the surface water layer and in the pycnocline-bottom layer (PBL) at six stations near the Danish coast between 1998 and 2012. Total annual PP and surface layer PP (SPP) correlate positively with landbased N-loading from Denmark (p , 0.003). The percentage of annual PP occurring in the PBL (denoted as deep primary production, DPP) varied annually between 6% and 30% (mean 5 17%). The absolute magnitude of the DPP, as well as its relative proportion of total water column PP, correlates negatively with N-loading (p , 0.009 and p , 0.0003, respectively). Thus, SPP decreases in response to decreased N-loading, while DPP increases. Land-based N-loadings also correlate positively with the light attenuation coefficient (R2 5 0.39, p , 0.05), which may in part explain the response in DPP to changes in N-loading. DPP occurs in active phytoplankton communities acclimated and/or adapted to low light and producing oxygen in the PBL water. Primary production (PP) is an important factor in structuring marine ecosystems, and changes in PP in response to increased nutrient loading have been identified as being responsible for symptoms of eutrophication in coastal marine systems (Nixon 1995; Cloern 2001; Smith 2003). After recognizing the relationship between anthropogenic nutrient loading and eutrophication, many countries have initiated programs to reduce nutrient enrichment (Boesch 2002) with the expectation that marine PP will respond to a reduction in land-based nutrient loading. In some areas, enough data have now been collected to allow researchers to examine the extent to which this expectation has been realized. The Baltic Sea Transition Zone (BSTZ), which comprises the Kattegat and the Belt Seas and thus forms the connection between the Baltic Sea and the Skagerrak, is such an area. It is a shallow, stratified, and temperate marine system with dynamic hydrography. Surface salinity varies in the region from 10 to 14 in the southern part of the region and from 20 to 25 in the northern Kattegat

Loss of water clarity is one of the consequences of coastal eutrophication. Efforts have therefore been made to reduce external nutrient loadings of coastal waters. This paper documents improvements to water clarity between 1985 and 2008–2009 at four stations in the microtidal estuary Roskilde Fjord and find significant relationships to freshwater nutrient loadings. The paper then investigates to which extent changes in phytoplankton biomass (chlorophyll a (Chl a)), non-algal particulate organic matter (POM*), and residual attenuation in the water (Kb), respectively, can account for this optical improvement. Vertical light attenuation (Kd) declined, on average, by 34 %, accompanying a 71 % reduction of Chl a and an 80 % reduction of POM*. Residual attenuation declined by 26 % over the period in accordance with a measured 34 % decline of dissolved organic nitrogen. Analysis of simultaneous changes in light attenuation and Secchi depth also suggested a reduction of the scatter-to-absorption ratio over time. Considering the stronger reductions of particle concentrations than dissolved organic matter, the contribution of residual attenuation to vertical attenuation increased from 54 to 74 % in 1985 to 78 to 85 % in 2008–2009. Overall, efforts to reduce nutrient loading and improve water clarity appeared to have had a larger impact on POM* than on Chl a and colored dissolved organic matter concentrations in the estuary, which can account for the decrease in the scatter-to-absorption ratio. These optical changes lead to larger improvements of Secchi depth than of vertical light attenuation. The consequence of this is an overestimation (0.45–1.48 m) of the predicted increase of potential seagrass depth limits when based on Secchi depth rather than Kd.

In the 1980s, Danish coastal waters suffered from eutrophication and several nutrient management plans have been implemented during the years to improve ecological status. This study aims at giving a holistic ecosystem perspective on 25 years of mitigation measures. We report trends of nutrient inputs and the responses to these in various chemical and biological components. Nutrient inputs from land were reduced by ~50 % for nitrogen (N) and 56 % for phosphorus (P) since 1990. These reductions resulted in significant and parallel declines in nutrient concentrations, and initiated a shift in the dominance of primary producers towards reduced phytoplankton biomass (chlorophyll a concentration) and increased cover of macroalgae in deeper waters. In the last 5 years, eelgrass meadows have also expanded towards deeper waters, in response to improving water clarity. An expected improvement of bottom water oxygen conditions has not been observed, presumably because more frequent stratification and higher water temperatures have counteracted the expected positive effects of reduced nutrient inputs. The biomass of the benthic macrofauna decreased as expected, but it was composed of a drastic decline of filter feeders paralleled by a more moderate increase of deposit feeders. This shift was most likely induced by increasing stratification. The reduced benthic filtration along with the limited eelgrass cover probably kept relatively more particles in suspension, which can explain why improvements in the Secchi depths were modest. Overall, several ecosystem components demonstrated clear signs of improvement, suggesting that at least partial recovery is attainable. On this basis, we propose a conceptual scheme for recovery of shallow coastal ecosystems following marked reductions in nutrient inputs.

Eelgrass depth limits and water clarity in the Skive Fjord estuarine system have not improved despite nutrient input reductions of 30%. Long-term monitoring data (1989–2010) were used to investigate the underlying causes. Dissolved inorganic and organic nitrogen concentrations decreased significantly over time, whereas particulate organic nitrogen concentration, assumed to consist primarily of phytoplankton and phytoplankton detritus and calculated as a proportional factor to chlorophyll a, did not change. Total organic carbon, mostly of autochthonous origin, remained constant despite reduced nitrogen concentrations, resulting in an increasing C:N ratio of the organic material in the water column. Phytoplankton primary production also remained constant suggesting that phytoplankton growth was only limited by nitrogen to a minor degree. Alleviated grazing pressure caused by a reduction in the blue mussel standing stock and a pelagic food web dominated by jellyfish may have contributed to the constantly high phytoplankton levels. Particulate inorganic matter, likely reflecting sediment resuspension, increased over time, most probably in response to removal of blue mussels and declining eelgrass cover. The Skive Fjord estuarine system is affected by multiple pressures—nutrient enrichment, mussel dredging and climate change that must be addressed together for water clarity to improve and eelgrass to recover.

The effects of temperature and food availability on feeding and egg production of the Arctic copepod Calanus hyperboreus were investigated in Disko Bay, western Greenland, from winter to spring 2009. The abundance of females in the near bottom layer and the egg production of C. hyperboreus prior to the spring bloom document that reproduction relies on lipid stores. The maximum in situ egg production (± SE) of 54 ± 8 eggs female −1 d −1 was recorded in mid-February at chlorophyll a concentrations below 0.1 µg l −1 , whereas no egg production was observed in mid-April when the spring bloom developed. After reproduction, the females migrated to the surface layer to exploit the bloom and refill their lipid stores. In 2 laboratory experiments, initiated before and during the spring bloom, mature females were kept with and without food at 5 different temperatures ranging from 0 to 10°C and the fecal pellet and egg production were monitored. Food had a clear effect on fecal pellet production but no effect on egg production, while temperature did not have an effect on egg or fecal pellet production in any of the experiments. Analyses of carbon and lipid content of the females before and after the experiments did not reflect any effect of food or temperature in the pre-bloom experiment, whereas in the bloom experiment a clear positive effect of food was detected in female biochemical profiles. The lack of a temperature response suggests a future warmer ocean could be unfavorable for C. hyperboreus compared to smaller Calanus spp. which are reported to exploit minor temperature elevations for increased egg production.

Benthic flora can contribute significantly to gross primary production (GPP) of shallow coastal waters where light reaches the sea bottom. We quantified and compared benthic and pelagic GPP along nutrient gradients in time and space in the shallow estuary, Limfjorden, Denmark, based on monitoring data combined with historical information. Limfjorden experienced a shift from a pristine, benthic-dominated clear water regime with high total GPP in the early twentieth century to a eutrophic, plankton-dominated regime still with high total GPP in the 1980s when nutrient loadings peaked. Recent reductions in nutrient loadings reduced pelagic GPP, particularly in spring, but water clarity and benthic GPP did not increase correspondingly, so total GPP declined. The most nutrient-rich basins have remained plankton-dominated, with benthic vegetation constrained to shallow waters. The results support existing evidence that total GPP of shallow coastal areas does not increase systematically with eutrophication. Furthermore, the results suggest that total GPP may decline temporarily during oligotrophication as pelagic GPP declines, while feedback mechanisms delay or prevent restoration to a state with benthic dominance of GPP.

The relative importance of nutrient inputs from streams and the open sea for the water quality is a key issue for an effective management of eutrophication problems in estuaries. In order to resolve this question we have developed a coupled 3D hydrodynamic–ecological model with numerical nitrogen tracking for Horsens estuary, which is a typical Danish estuary that is shallow and well ventilated with water from the adjacent open sea. The nitrogen tracking technique revealed that nitrogen discharges from local streams accounted for the major part (40%–90%) of the nitrogen in NO3, NH4 and detritus whereas only 15% to 50% of the nitrogen in phytoplankton originated from streams. Effects of nitrogen loading reductions were most pronounced for nutrient concentrations and Chl a concentrations whereas the light condition was only slightly improved. The impact of stream nitrogen reductions on nitrate concentrations, Chl a concentrations and light attenuation was respectively 7, 2 and 3 times higher than the impact of open sea nitrogen reductions. Hence, even in estuaries that are well ventilated with water from the adjacent sea significant environmental improvements can be expected after reduction in local nutrient discharges.

The onset of spring bloom in temperate areas is a transition period where the low productive, winter phytoplankton community is transformed into a high productive spring community. Downwelling irradiance, mixing depth and the ability of the phytoplankton community to utilize the light, are key parameters determining the timing of the onset of the spring bloom. Knowing these parameters would thus provide tools for modeling the spring bloom and enhance our knowledge of ecophysiological processes during this period. Our main objective with this study was to provide data for the growth characteristics of some key species forming the spring bloom in the Gulf of Finland, and to apply those results in a simple dynamic model for the onset of the spring bloom, in order to test if the timing of the spring bloom predicted by the models corresponds to field observations. We investigated the photosynthetic characteristics of three diatoms and two dinoflagellates ( Chaetoceros wighamii , Melosira arctica , Thalassiosira baltica , Scrippsiella hangoei and Woloszynskia halophila ), at low temperatures (4–5 °C). All of these species are common during spring bloom in the Baltic Sea. Cultures of these species were acclimated to different irradiance regimes prior to measurements of photosynthesis, respiration, pigment concentration and light absorption. We did not find a positive relationship between respiration and growth rate, and we hypothesize that this relationship, which is well established at higher temperatures, is negligible or absent at low temperatures ( P m ), and maximum light utilization coefficient ( α ) was lowest and respiration ( R ) highest in the dinoflagellates. We made a model of the onset of the spring bloom in the western part of Gulf of Finland, using the obtained data together with monitoring data of mixing depth and water transparency from this area. Model results were compared to field observations of chlorophyll- a (Chl- a ) concentration. There was a good agreement between the model predictions and the observed onset of the spring bloom for the diatoms. S. hangoei , however, was not able to reach positive production in the model, and W. halophila had the similar growth characteristics as S. hangoei . Consequently, these species must have other competition strategies enabling them to exist and grow during spring bloom.

Scattering alters the path of photons, but ultimately their removal from the water column occurs by absorption by one of four components: water itself, coloured dissolved organic matter (CDOM), phytoplankton pigments or non-algal particulate matter (NAP). We calculated absorption budgets for two fjordal systems, Godthabsfjord (SW Greenland) and Young Sound (NE Greenland), based on these components and evaluated the fate of solar radiation in each system. Absorption by phytoplankton pigment accounted for 15–32% of photons in Godthabsfjord whilst in Young Sound the corresponding fraction was only 5–8%. NAP accounted for 13–25% of absorption in Young Sound and only 7–8% in Godthabsfjord whilst fractions of absorption by CDOM were more similar: 6–13% in Godthabsfjord and 6–18% in Young Sound. In typical temperate estuarine systems, nutrients, CDOM and particulate matter are supplied by riverine sources. Increased nutrient supply will tend to increase productivity whilst increased concentrations of CDOM and particles will increase light attenuation, thereby reducing productivity. The two Greenlandic fjords differ from typical estuarine systems in that their supply of nutrients and particles is decoupled. Freshwater feeding them comes from glacial melt and contains particles but low concentrations of nutrients. New nutrients are supplied by entrainment of oceanic water at the mouths of the fjords and sediment–water exchange of remineralized nutrients. Attenuation and thereby light availability in the two fjords is strongly correlated with turbidity (Godthabsfjord: r2 = 0.90 p

Processes governing productivity at the base of the pelagic food web of the southern Indian Ocean are influenced primarily by physical‐chemical conditions with implications for the structure and function of the entire pelagic food web. Here, we report observations along a great circle transect from Cape Town, South Africa, to Broome in north western Australia. Primary production was tightly linked to water column stability and nutrient availability, with high productivity (1144 mg C m 22 day 21 ) in the sub-tropical convergence zone, and falling off by an order of magnitude in the sub-tropical gyre and tropical waters off northwest Australia. Primary production was largely confined to the GF/F fraction (GF/F .75% of total production and usually much higher) and the photosynthetic parameters showed adaption to the prevailing light levels. Bacterial production ranged from 19 to 155 mg C m 22 day 21 . No relationships were found between bacterial production and primary production or phytoplankton biomass and bacterial production seem more related to the state of the phytoplankton community as high bacterial production was observed in a post-bloom situation. The average BP:PP ratio was 31% (range 3.5‐71%).

We present the results of a mesocosm experiment investigating the production and utilization of autochthonous dissolved organic matter (DOM) by the plankton community under different inorganic nutrient regimes. Fluorescence spectroscopy combined with parallel factor analysis was applied to study the dynamics of autochthonous DOM. Seven independent fluorescent fractions were identified, differing in their spectral characteristics, production rates, and sensitivity to photochemical and microbial degradation processes. Five different humic fractions, a marine protein, and a peptide fluorescence were found. The five humic fractions were produced microbially, with the greatest production occurring under combined Si- and P-limiting conditions. The two proteinaceous fractions were produced during exponential growth of phytoplankton, irrespective of biomass composition. Photodegradation was an important sink for the microbially derived humic material, and the marine protein material was susceptible to both photoand microbial degradation. The amount of carbon bound in dissolved organic matter (DOM) in the world’s oceans is similar to that bound as atmospheric carbon dioxide (Siegenthaler and Sarmiento 1993). As a result of increasing interest in the global carbon cycle, research of DOM has intensified over the last 30 yr. Although the supply of terrestrially derived DOM to the world’s oceans is considerable and it often dominates coastal seas, it is thought to represent only about 2‐3% of the total oceanic DOM pool (Opsahl and Benner 1997). The majority of marine DOM is therefore autochthonous (i.e., produced in the marine environment by phytoplankton). The major sources of autochthonous DOM include extracellular release by phytoplankton, release by grazers, and viral lysis of plankton (Nagata 2000). A large fraction of the DOM produced by these processes is consumed and respired rapidly by microbial activity; as a result, it can be difficult to measure. A smaller, more refractory fraction accumulates in seawater and is degraded over longer timescales (Nagata 2000). This fraction consists of both large, complex molecules (humic material) formed by condensation reactions and lower molecular weight organic matter from bacterial cell structures (Harvey et al. 1983; Nagata 2000). DOM consists of a complex mixture of compounds as a result of its variety of sources and continual reworking by photochemical and microbial degradation processes (Scully 1

Excitation emission matrix fluorescence spectroscopy combined with PARAFAC analysis provides a fast and effective method of characterizing the fluorescent fraction of dissolved organic matter (DOM). Fluorescence measurements can be used as a tracer for quantitative and qualitative changes occurring in the DOM pool as a whole. An earlier study found that the fluorescence signal could be modeled by five fractions. This study presents an analysis on a considerably larger data set (>1,200 samples) resulting from a 1-yr sampling program in Horsens Estuary, Denmark. Eight fluorescent fractions were identified. Four biogenic terrestrial, two anthropogenic, and two protein-like fractions were identified. Analysis of covariation between the components identified source-specific fractions and the presence of common factors controlling the composition of terrestrial DOM exported from different catchments.

The influence of phytoplankton photoacclimation and adaptation to natural growth conditions on the chlorophyll a-specific in vivo absorption coefficient (a* ph ) was evaluated for samples collected in estuarine, coastal and oceanic waters. Despite an overall gradient in the physio-chemical environment from estuaries, over coastal, to oceanic waters, no clear relationships were found between a* ph and the prevailing light, temperature, salinity and nutrient concentrations, indicating that short-term cellular acclimation was of minor importance for the observed variability in a* ph . The clear decline in a* ph from oceanic, over coastal, to estuarine waters was, however, strongly correlated with an increase in cell size and intracellular chlorophyll a (chl a) content of the phytoplankton, and a reduction of photosynthetic carotenoids relative to chl a. Variations in photoprotective carotenoids relative to chl a seemed to be of minor importance for the variability in a* ph . In addition, significant differences in phytoplankton composition and abundance were observed, primarily driven by an increase in the abundance of diatoms, which furthermore correlated with increasing pigment packaging and decreasing a* ph . The observed differences in a* ph were, therefore, primarily driven by longerterm adaptations of the phytoplankton community. Our data suggests that an overall increase in nutrient loading from oceanic to estuarine waters increases phytoplankton abundance and favors larger sized species, particularly within the diatoms. These changes eventually decrease a* ph through a rise in the package effect and a lower abundance of photosynthetic carotenoids relative to chl a.

The optical properties of coloured dissolved organic matter (CDOM) can be used, in some environments, to trace water masses and provide information about the dynamics of the dissolved organic fraction in natural waters. This work presents the results from a modelling exercise, laboratory experiment and field data, which describe the variations in the optical properties of CDOM during mixing. The exponential slope coefficient (S) is frequently used to characterise different CDOM pools; however, its behaviour during conservative mixing of two different CDOM types is often misunderstood. Identification of a theoretical conservative mixing line allows the rapid identification of non-conservative processes (e.g. in situ production, flocculation and degradation) acting on the pool during mixing. The results suggest that some of the patterns reported in the literature could purely be a result of conservative mixing rather than a product of non-conservative processes.

A spectral matching parameter (absorption efficiency, Ae) was developed to quantify the relationship between the light absorption spectra of phytoplankton communities and the spectral irradiance of their ambient light field. Ae was defined as the ratio between the amount of radiation absorbed by the phytoplankton in situ and the amount absorbed in a spectrally flat light regime. This approach was applied to our measurements of spectral absorption for the phytoplankton communities in six lakes in High Arctic Canada that spanned a range of bio-optical conditions. Ae values were calculated for the light spectrum down through the water column and for 11 types of artificial light source. Spectral matching varied among lakes and with depth. There was a significant linear relationship between the relative change in Ae with depth and the diffuse attenuation coefficient Kd (r 2 = 0.52, P = 0.012 for Kd for the 400‐700 nm waveband; r 2 = 0.78, P = 0.0003 for Kd at 440 nm). The tabulated values for the matching parameter Ae allow the comparison of photosynthesis versus irradiance (P versus E) curves among studies using different light sources. Ae estimates also facilitate the evaluation of chromatic adaptation in natural waters, and the calculation of spectrally adjusted, in situ primary production down through a water column from P versus E relationships under a single spectral regime.

The optics of chromophoric dissolved organic matter (CDOM) in the Greenland Sea were investigated and compared to results from earlier studies in the Southeastern North Sea. Absorption at 375 nm (a375) in the Greenland Sea varied from 0.77 m−1 to the detection limit of our instrument (0.05 m−1), with the highest values found during summer. The spectral slope coefficient (S) ranged from 8.2 to 26.4 µm−1 with the highest values occurring during winter. Seasonal variations in the in situ production and degradation of CDOM where shown to be responsible for the trends seen. A negative correlation between S and a375 was evident in the Greenland Sea and differed noticeably from that found in coastal waters. The differing S-a375 behavior of CDOM known to be of terrestrial origin allowed the development of an algorithm for the differentiation between marine and terrestrial organic matter. The behavior of marine CDOM was modeled by S = 7.4 + 1.1/a375.

The spectral pattern of light attenuation in the ultraviolet (UV) and blue region (360‐500 nm) was analyzed for seven high Arctic lakes. The best description of Kd versus wavelength was obtained with an exponential model similar to the model used for absorption by chromophoric dissolved organic compounds (CDOM), but with an additional parameter (Kback) that accounts for background scattering: S(l 2l) 0 K (l) 5 K (l )e 1 K , dd 0 back where Kd(l) is the diffuse vertical attenuation coefficient at wavelength l and S is the exponential slope parameter that characterizes the decrease in attenuation with increasing wavelength. The inclusion of the background parameter gave a significantly better fit and eliminated the systematic deviations over the spectrum that occur in the absence of Kback. The resultant S values (mean 5 17.4 mm 21 ) were on average 47% higher than values calculated without a background parameter, and were more sensitive to between-lake differences. The same pattern was found when S was estimated from spectral Kd values in three literature data sets, so the new equation will generally increase the estimated values of S based on Kd and provide a more accurate guide to intersystem variability. A compilation of literature data for S (based on Kd and the new equation or on absorbance) showed that its mean value (6SE) is significantly higher in freshwaters (17.1 6 0.7 mm 21 ) than in the sea (14.0 6 0.4 mm 21 ). The variability in S was highest for low values of Kd or a (340 nm values below 3 m 21 ). This pattern may be due to instrument-related problems or more likely represents the real variability in the optical properties of CDOM in low colored systems. The analysis showed that S can vary significantly between and within systems (overall range 5 9.2‐36.2 mm 21 ) and that the value obtained also depends on the method of calculation, the wavelength range, and the type of optical measurement.

1. The underwater light climate and benthic moss communities of Grane Langso were investigated in May 1997 to determine the potential effects on benthic production of changing water column attenuation and spectral quality of light. 2. A reduction in water clarity in the lake since the 1960s was manifested as a marked increase in the attenuation of blue light, relative to red light, which can be attributed to increased dissolved organic carbon. 3. The biomass of the benthic moss community ranged from a maximum of 195 gDW m−2 at a depth of 4 m to 39 g DW m−2 at a depth of 10 m and Drepanocladus exannulatus contributed 70% of the biomass at all depths. 4. Absorption of PAR by D. exannulatus was maximal in the highly pigmented youngest parts of the plant and these correspondingly showed the highest rates of net photosynthesis. The absolute amount of chlorophyll-a per g dry weight was greater at 10 m than 2 m, but the ratio of accessory pigments to chlorophyll-a did not change. Deep growing plants did not show adaptation to changed light quality. 5. Increased attenuation of blue light and the resultant overall decrease in water clarity is likely to impact negatively on net annual production of benthic macrophytes of Grane Langso. Any further increase in dissolved organic carbon concentration has the potential to markedly decrease the depth to which mosses grow by reducing the length of time in a year during which net photosynthesis occurs.

Photosynthesis vs. irradiance relationships were determined for phytoplankton communities from seven lakes in the Canadian high Arctic, including ultraoligotrophic Char Lake, nutrient-enriched Meretta Lake, and two meromictic lakes. The derived photosynthetic parameters were low for all samples, with a mean (6SD) light-saturated photosynthetic rate (P ) of 0.46 (60.28) g C g 21 chlorophyll a (Chl a )h 21 and a mean a B (light-limitation parameter) B m of 1.23 (60.56) g C g 21 Chl a m 2 mol 21 . The saturation irradiance (Ek) ranged from 50 to 196 mmol quanta m 22 s 21 and was positively correlated with mean irradiance for the water column. Quantum yields for photosynthesis in the Arctic lake phytoplankton were also low (mostly ,10 mmol C mol 21 quanta). An intersystem comparison of a B and P values with literature data for algae from other cold environments showed that the photosynthetic B m parameters for phytoplankton in Arctic and Antarctic lakes are three- to sixfold lower than for marine algae, ice algae, and cultures over the same low-temperature range. This may be the result of more severe nutrient stress in high-latitude lakes relative to polar marine environments and to the persistence of nonactive pigments in cold freshwaters.

Aquatic mosses are often the exclusive form of macrophytic vegetation in Arctic lakes. Despite the cold nutrient-poor water and the short ice-free summer, the mosses form dense stands on the lake bottom down to great depths. The environmental conditions suggest that moss growth and decomposition are extremely slow, but logistical and methodological difficulties have so far precluded direct measurements of the processes. Here, we use temporal changes in the size and density of leaves along the axis of moss shoots collected from different depths in Char Lake and North Lake in the Canadian High Arctic to reconstruct the annual growth and decomposition of the mosses during the past 10-17 years. Our results show low but remarkably constant annual elongation rates (about 10 mm·shoot-1) in the long-lived shoots that carry green leaves for several years and decompose slowly. Cold temperatures and low nutrient supply in combination with the short Arctic growing season can account for the low growth rate, the low decomposition rates, and the unprecedented longevity of these moss communities relative to other submerged macrophytes.

1. Data for maximum colonization depth (Zc) of five groups of submerged macrophytes and light attenuation were collected for forty-five Danish lakes and 108 non-Danish lakes. The macrophyte groups were bryophytes, charophytes, caulescent angiosperms, rosette-type angiosperms and Isoetes spp. 2. The data showed systematic differences among the groups in the relationship of Zc to water transparency. In lakes with low transparency (Secchi disc transparency (Zs) less than 7 m) caulescent angiosperms and charophytes penetrated deepest followed by bryophytes and Isoetes spp. In more transparent lakes bryophytes grew deepest, followed by charophytes, caulescent angiosperms and Isoetes spp. Rosette-type angiosperms had the lowest Zc in all types of lakes. Charophytes and caulescent angiosperms had similar depth limits in lakes with Zs < 4 m but charophytes grew deeper in more transparent lakes. The depth limits of both groups were independent of light penetration in lakes with very low transparency (Zs < 1 m). The annual light exposure for the deepest growing macrophytes (bryophytes) was 20–95 mol photons m–2. 3. The relationship between Zc, macrophyte type and lake transparency could be explained by three distinct processes regulating Zc. In lakes with low transparency (Zs < 1 m), tall macrophytes (caulescent angiosperms and charophytes) compensate for light limitation by shoot growth towards the water surface and Zc is therefore independent of transparency. In lakes with medium transparency (1 m < Zs < 4 m) Zc for angiosperms, charophytes and Isoetes spp. is constrained by light attenuation in the water column, corresponding to a linear relationship between Zc and Zs. This pattern also applies to bryophytes, despite lake transparency. In transparent lakes, the minimum light requirement at Zc increased with increasing transparency for angiosperms, charophytes and Isoetes spp. 4. The minimum light requirements among submersed macrophytes (including marine macroalgae) depend on their plant-specific carbon value (plant biomass per unit of light-absorbing surface area) for the species/group, indicating that the light requirements of submersed plants are tightly coupled to the plants’ possibility to harvest light and hence to the growth form. 5. The light requirements increased on average 0.04% surface irradiance per degree increase in latitude corresponding to an average decrease in Zc of 0.12 m per degree latitude.

We have tested the importance of thallus thickness in marine macroalgae for the growth-irradiance relationship measured in the laboratory and for the maximum growth rate measured in the field. We found large differences in the growth-irradiance relationship depending on the biomass parameter (surface area, fresh weight, carbon content) applied. Use of carbon content yielded meaningful reproducible patterns and it appeared to be the most suitable descriptor of plant energetics. Surface area and fresh weight increased in the dark despite loss of carbon in the plant tissue. The growth kinetic parameters were closely related to thallus thickness, here described as thallus-specific carbon (TSC, mol tissue C m−2 surface area). Both the loss rates in the dark and the growth rates in carbon biomass at low and high irradiance decreased with increasing TSC of the species. The decline in growth rate with increasing TSC was described by the allometric equation log(growth rate) = a + b × log(TSC). The slope, b, was be...

The concentration of labile dissolved organic C ([DOC,]) and the plankton community structure were measured weekly during a diatom spring bloom and the subsequent clear-water phase in a eutrophic, temperate lake. The DOC, concentration was assessed by a bacterial regrowth method based on direct measurements of the increase in bacterial biomass and its respiratory demand. [DOC,] ranged from 560 to 1,130 pg C liter - I and accounted for only 5-9% of total DOC. DOCL oscillated around 800 pg C liter - I, and the amplitude over 7-d periods was within k 16% of the in situ bacterial carbon demand. One period (3 weeks) with consistently increasing [DOC,] took place concomitant with the increase in macrozooplankton and bacterial biomass. Thus, DOC, was positively related to decreases in chlorophyll and zooplankton grazing. The increase in DO& was followed by 3 weeks with decreasing concentrations, when the relationship between DOC, and bacterial production was negative. This pattern gives support to the hypothesis that grazing is a,quantitatively important process in the flow of carbon to bacteria. The variations of DOC, were analyzed with a model describing interactions among substrate, bacteria, and grazing. Short-term (days) oscillations probably result from pulses of substrate production that are followed within a few days by bacterial uptake, and long-term (weeks) variations are explained by a variable bacterial substrate affinity (K,,).

The effects of temperature and food was examined for Calanus finmarchicus and C. glacialis during 3 phases of the phytoplankton spring bloom in Disko Bay, western Greenland. The 2 species were collected during pre-bloom, bloom, and post-bloom and exposed to tempera- tures from 0 to 10°C, combined with deficient or excess food. Fecal pellet and egg production were measured as indices for grazing and secondary production, respectively. Furthermore, changes in body carbon, nitrogen, and lipid content were measured. C. glacialis sampled before the bloom and incubated with excess food exhibited high specific egg production at temperatures between 0 and 2.5°C. Higher temperatures did not increase egg production considerably, whereas egg pro- duction for C. finmarchicus more than tripled between 2.5 and 5°C. Starved C. glacialis produced eggs at all temperatures stimulated by increasing temperatures, whereas starved C. finmarchicus needed temperatures above 5°C to produce eggs fueled by their lipid stores. Few C. finmarchicus had mature gonads at the initiation of the pre-bloom and bloom experiment, and egg production of C. finmarchicus therefore only increased as the ratio of individuals with mature gonads increased. During the bloom, both C. glacialis and C. finmarchicus used the high food availability for egg production, while refueling or exhausting their lipid stores, respectively. Finally, during the post-bloom experiment, production was low by C. finmarchicus, whereas C. glacialis had terminated production. Our results suggest that a future warmer ocean will reduce the advantage of early spawning by C. glacialis and that C. finmarchicus will become increasingly prevalent.

This study presents the results of a year-long study investigating the characteristics of dissolved organic matter (DOM) in the Danish estuary, Horsens Fjord. The estuary is shallow with a mean depth of 2.9 m and receives high loadings of inorganic nutrients from its catchment. The behaviour of different DOM parameters i.e. dissolved organic carbon (DOC), nitrogen (DON), and phosphorous (DOP), light absorption and eight fluorescence components, were analysed relative to conservative mixing. Many of the parameters did not behave conservatively. For DON, DOP and absorption, more than 65% of the freshwater concentration was removed initially at salinities below 12. At higher salinities two general patterns were identified. Concentrations of DON, DOP and four humic fluorescent fractions were not, or only weakly, related to salinity, showing that other processes than mixing were involved. Other parameters such as DOC and two terrestrial humic components behaved conservatively. The same was true for DON during winter. These results are consistent with the finding that autochthonous DOM was the dominant source of DOM in this estuary. The molar C:N and C:P ratios for DOM (DOC:DON and DOC:DOP) in freshwater were 11 and 758, respectively. The DOC:DON ratio increased in the estuary during the productive season to average values between 13 and 17, due to accumulation of DOC and removal of DON. The DOC:DOP ratio decreased within the estuary showing that in general DOM was enriched with phosphorous, however, during the spring, when phosphorous was limiting, the DOC:DOP ratio increased due to low DOP concentrations. We hypothesise that in estuaries with high loadings of inorganic nutrients relative to DOM, production and degradation of DOM within the estuary will dominate over allochthonous inputs and control both concentration and characteristics of DOM. A conceptual model for this hypothesis is presented.

Light utilization efficiency in five species of marine macroalgae was measured in laboratory growth experiments (13–41 days duration) at different irradiances at 7°C. All species acclimated to irradiance by changing their light absorption, resulting in a peak in light absorption between 2 and 15 μmol·m−2.s−1. Light absorption increased with thallus-specific chlorophyll and carbon content according to linear inverse relationships between chlorophyll content (chlarea−1) and log[transmission] and between log[carbon content, Carea−1] and log[transmission]. Quantum yields for light-limited growth and estimated gross photosynthesis were calculated based on incident and absorbed light. Quantum yield for photosynthesis based on light absorbed by pigments was high (mean = 114 mmol C·mol−1 photons) and similar among the species. Quantum yield for net growth based on incident light was also high but more variable, between 22 and 75 mmol C·mol−1 photons. Differences among species were mainly due to differences in light absorption. In conclusion, all species acclimated to low light by increasing light absorption to the maximum attainable, and growth efficiencies based on absorbed light were close to the maximum theoretically possible.

Relationships between key phytoplankton attributes including Chl a-specific light absorption, pigment composition and concentration, photosynthesis, primary production and community structure were studied in two open shallow nutrient-poor coastal systems receiving similar amounts of sewage water. Both systems were significantly nitrogen limited. However, differences in wastewater treatment (primary vs secondary) and sewage dilution (50%) between the two systems caused a greater difference between systems than locally around the outflows. For both systems, water at the outlet had significantly lower water transparency caused by a 20% higher absorption by coloured dissolved organic matter. Nutrient concentrations were also elevated, gradually decreasing with distance north (governing current) of the outflows, causing higher abundance of nano-sized phytoplankton, higher content of carotenoid pigments, 20–50% higher Chl a-specific absorption coefficients and higher photosynthetic capacity. Although maximum rates of Chl a-normalised photosynthesis were strongly related to nitrate availability, no effects were found on the derived areal primary production or algal biomass suggesting that photosynthetic and optical parameters are more sensitive indicators of nutrient enrichment than biomass or productivity.

The effect of external glucose (51 mM) and acetate (13 mM) on growth and photosynthetic capacity of Ulva lactuca L. was tested in laboratory cultures over 41 days in the dark and in dim light (0.9 μmol photons·m−2·s−1) at 7–8° C. Glucose and acetate had a significant positive effect on growth rate, chlorophyll content, and quantum yield for discs grown in the dark and in dim light. The carbon gain from heterotrophic uptake was low and only allowed U. lactuca to maintain a specific uptake was low and only allowed U. lactuca to maintain a specific growth rate of 0.005 day−1 compared to 0.06–0.1 day−1 at higher light intensities. However, plants with added organic substrate maintained a normal chlorophyll content and were able to photosynthesize whereas control plants lost pigmentation and photosynthetic capability after 41 days in both dim light and darkness, probably because of disorganization of the photosynthetic apparatus. This suggest that the ecological significance of heterotrophic uptake is to allow U. lactuca to survive during prolonged low light conditions with an intact photosynthetic apparatus.

We investigated the partitioning of carbon, nitrogen, and phosphorus between particulate and dissolved production using 11-m3 marine mesocosms (bags) in a Norwegian fjord with a salinity of 28.3, a chlorophyll concentration of 0.6 m gL 21, an even biomass among five algal groups, and nitrogen limitation as the initial conditions. The experiment lasted 21 days in August. Addition of silicate (+Si) resulted in diatom dominance, while a more diverse community was present in treatments with no added Si (2Si). Addition of inorganic nutrients in a N : P gradient from 64 to 4 either conserved the initial N limitation or forced the plankton communities to P limitation. Per added limiting nutrient, the diatom-dominated bags produced more particulate (POC) and dissolved organic carbon (DOC) than the other bags. However, the relative partitioning of net production to POC and DOC did not differ as a function of the plankton communities. Between 22% and 33% of the net production accumulated as new DOC. The higher values were found in the N-limited bags. The production of new dissolved organic nitrogen (DON) was variable over time, and short periods of positive production were followed by removal (negative production). Between 6% and 22% of the assimilated N was

The study establishes an annual estimate for annual primary production of 81 g C m −2 for the open Greenland Sea based on data from five cruises and literature data. This estimate agrees well with a model estimate based on nutrient utilisation but is a factor of 2–5 less than published primary production estimates made by remote sensing of this area. The seasonal distribution of particulate primary production in open Greenland Sea waters followed the seasonal distribution of surface irradiance with a peak in June, indicating that light is the primary factor governing primary production in the area. At stations along the ice edge, blooms were recorded in both June and August, suggesting a pattern of repeated blooms during the summer season at the ice edge. Subsurface phytoplankton peaks were a persistent feature in the open Greenland Sea from May to August. These peaks were consisted of actively photosynthesising phytoplankton and up to 90% of total water column particulate primary production was estimated to occur in association with these peaks. Diatoms dominated the phytoplankton community during the spring bloom and in the Polar Water during August. Size distribution analyses of the phytoplankton communities indicated that the relative abundance of large cells compared to small cells was greatest in May as compared to June and August. No significant differences were noted between June and August in the slope of the phytoplankton size distribution spectra. Inorganic nitrogen and phosphorus nutrients were measurable in surface waters on all cruises. Only in August were there some indications (altered Redfield ratios and higher nutrient concentrations in subsurface chlorophyll peaks than at the surface) of nutrient depletion of surface waters. Implications for food web structure and carbon flux of these patterns in phytoplankton activity and distribution are discussed.

Nutrient over-enrichment and cultural eutrophication are significant problems in the Danish marine environment. Symptoms of eutrophication include periods of hypoxia and anoxia in bottom waters, death of benthic-dwelling organisms during anoxia, long-term reductions in the depth distribution of macrophyte communities, changes in the species composition of macrophyte communities, and increases in reports of harmful algal blooms. In 1987 the Action Plan on the Aquatic Environment was adopted to combat nutrient pollution of the aquatic environment with the overall goal of reducing nitrogen loads by 50% and point source phosphorus loads by 80%. The Danish Aquatic Nation-wide Monitoring Program was begun in 1988 in order to describe the status of point sources (industry, sewage treatment plants, stormwater outfalls, scattered dwellings, and fish farms), ground water, springs, agricultural watersheds, streams, lakes, atmospheric deposition, and the marine environment. Another important aspect of the program was to document the effects on the aquatic environment of the measures and investments taken for nutrient reduction as outlined in the Action Plan. The monitoring program should determine if reductions in nutrients are achieved by the measures taken and should help decision makers choose appropriate additional measures to fulfill the objectives. Coordination with international programs and commissions is an important component of the monitoring program to meet internationally agreed upon reductions in nutrient inputs. The future and direction of the Danish National Aquatic Monitoring and Assessment Program will be to a large extent shaped by both the Water Framework Directive and Habitat Directive adopted by the European Union.

The optical properties of chromophoric dissolved organic matter (CDOM) in Danish estuaries and coastal waters were investigated. A new method for estimating the spectral slope coefficient (S) was examined and found to give a closer fit for the measured absorption (92% reduction in sum of residuals) than the traditional method. The spectral pattern in residuals produced by the new technique enabled the identification of CDOM originating from areas of different land uses/types where S coefficients were similar. S values were found to behave conservatively with respect to salinity in all waters except for the off-shore North Sea region where CDOM from marine sources was suspected to have more influence. The specific absorption coefficient of CDOM did not vary significantly in Danish coastal waters. These waters are of particular interest in bio-optical studies as they allow the study of CDOM across a gradient from estuarine, brackish to near-oceanic environments. The results demonstrate that it is possible to develop regional models for the optical properties of CDOM which would then allow higher precision in remote sensing applications.

Although seagrass detritus is exposed to high solar rabation in its natural environment, the impact of solar radiation has not yet been taken into account in degradation of particulate detritus We exposed sterilised detrital leaves of eelgrass Zostera marina L. to solar radiation. Under solar radiation the chlorophyll in leaves bleached in < 5 d and other absorbing compounds within 1 mo. During a 1 mo period the organic matter content of leaves (32 g m-2 of leaves) decreased, mainly via leaching, 3 to 9%) In darkness and 23 to 36';' under solar radiation. First order kinetlcs described the loss of organic matter as a function of cumulative global radiation well. The decay coefficient was 0.00038 MJ-' ,m2 Solar radiation-induced changes in the structure of leaves increased the bioavailability of detritus to bacteria 2to 3-fold. Our results show that solar radiation is one of the major factors in the decomposition of seagrasses and has to be taken into account in degradat~on models of particulate detritus.

Practical guidelines for the analysis of seawater Edited by Oliver Wurl Boca Raton, FL, CRC Press, Taylor & Francis Group, 2009, 401 pp., ISBN 978-1420073065. US$ 149.95 (hardback) The title promis...

12 pages, 5 figures, 1 table, The dark respiration of a natural plankton community from an eutrophic lake was studied in a laboratory scale enclosure (LSE), exposed to illumination which simulated natural light conditions in the water column. The dark respiration was measured continuously for 2 hours in samples obtained from the LSE each hour for 26 hours. The relationships between dark respiration rates, carbohydrate concentrations and other parameters were investigated. The dark respiration rate showed an exponential decrease with time in the dark in all light period incubations with a time coefficient of 0.3 h-1. The decrease in respiration rate in the dark period was much slower, reaching an approximately constant level at the end of the night. The overall dark period decline in respiration rate also exhibited an exponential pattern, but with a much lower time coefficient (0.04 h -1) than for the light period incubations. A linear relationship was found between dark respiration rate and carbohydrate concentration at night time but no relationship was apparent during the day. A comparison between these data and data from the literature show that this pattern of dark respiration rate decrease with time in the dark may have some general applications for dense phytoplankton communities. © 1992 Kluwer Academic Publishers

Inputs of dissolved carbon, nitrogen, and phosphorus were assessed for an estuary and its catchment (Horsens, Denmark). Seasonal patterns in the concentrations of DOM in the freshwater supply to the estuary differed depending on the soil and drainage characteristics of the area. In streams draining more natural areas the, patterns observed were largely driven by seasonal temperature fluctuations. The material exported from agricultural areas was more variable and largely controlled by precipitation events. Positive exponential relationships were found between the nitrogen and phosphorus loading, and the percentage of catchment area used for agriculture. Colored DOM (CDOM) loading measurements were found to be a good predictor of dissolved organic carbon (DOC) loading across the different subcatchments, offering a rapid and inexpensive alternative of operationally monitoring DOC export. For all the dissolved nutrient inputs to the estuary, dissolved inorganic nitrogen (DIN) and dissolved organic phosphorus dominated the loadings. Although 81% of the nitrogen annually supplied to the estuary was DIN, 83% of the nitrogen exported from the estuary was dissolved organic nitrogen (DON). Results show that increasing the area of the catchment covered by forest and natural pastures would have a positive effect on the trophic status of the estuary, leading to a considerable decrease in the phosphorus loading and a shift in the nitrogen loading from DIN to DON. Such a change in land use would also increase the export of DOC and CDOM to the estuary having the potential to increase oxygen consumption and reduce the photic depth.

Photoacclimation to different light and nutrient regimes was studied in 4 common marine phytoplankton species from northern European coastal waters. Significant changes were observed for all species in their cellular nutrient content, pigment packaging and composition, and chl a-specific in vivo light absorption (a* chla (λ)). From exponential to stationary growth, the C:N and carotenoid:chl a ratios increased, while the cellular chl a content decreased. All phytoplankton species acclimated to increasing irradiance by reducing their cellular chl a content and increasing the carotenoid:chl a ratio, resulting in higher chl a-specific absorption coefficients (a* chla ). Similar results were observed for N-depleted cultures. Although acclimation to different irradiance and nutrient conditions introduced a substantial variability in pigment composition and intracellular pigment concentration, significant interspecies differences in a* chla were still apparent. The interspecies differences were primarily due to differences in cell size, which covaried with pigment packaging and composition. On this basis, we suggest that optical acclimation (i.e. changes in pigment composition and packaging) to the prevailing growth conditions (light, temperature and nutrient availability) is less important than differences in phytoplankton composition (dominant cell size and pigment composition) for the variation in a* chla (λ) of natural phytoplankton communities.