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Nitrogen (N) limitation of primary production is common in temperate salt marshes, even though conservative N recycling can fulfill a large proportion of plant N demand. In nutrient poor young marshes, N limitation may be more severe and new N sources, such as N fixation, more important for plant growth. We measured N fixation and the response of salt marsh primary producers (Spartina alterniflora and benthic microalgae) to N fertilization in one mature (>150 year) and two young (7 and 15 year) naturally developing marshes at the Virginia Coast Reserve LTER site. S. alterniflora aboveground biomass in the mature marsh (1,700+/-273 g m(-2)) was 1.8 and 2.8 times higher than in the 15 year and 7 year old marshes, respectively. Fertilization significantly increased S. alterniflora biomass in the two young marshes (160-175%) and areal aboveground tissue N in the youngest marsh (260%). Microalgal chlorophyll a (Chl a) in the mature marsh was nearly 2-fold lower than in the 7-year-old marsh, and there was no evidence that this was due to light limitation. However, Chl a in fertilized plots was 30% higher than control plots at the youngest site. Daily N fixation decreased with increasing marsh age in summer, when rates were highest at all sites. Autotrophic N fixation (difference between rates in the light and dark) was most important in the summer, but we saw no indication of a shift in dominance between autotrophic and heterotrophic N fixers during marsh development. Estimated annual N fixation was 2- to 3-fold higher in the young marsh (18.3+/-1.5 g N m(-2) year(-1)), than in the intermediate-aged (9.0+/-0.7) or mature marsh (6.1+/-0.5). In the young marshes, N fixation was sufficient to provide a substantial proportion of aboveground S. alterniflora N demand. Our results suggest that both benthic microalgae and S. alterniflora in young salt marshes are N limited, and that this limitation decreases as the marsh matures. The high rates of N fixation by autotrophic and heterotrophic bacteria in the sediment could provide an important source of N for primary producers during marsh development.

Macroalgae, often the dominant primary producers in shallow estuaries, can be important regulators of nitrogen (N) cycling. Like phytoplankton, actively growing macroalgae release N to the water column; yet little is known about the quantity or nature of this release. Using 15N labeling in laboratory and field experiments, we estimated the quantity of N released relative to assimilation and gross uptake by Gracilaria vermiculophylla (Ohmi) Papenfuss (Rhodophyta, Gracilariales), a non-native macroalgae. Field experiments were carried out in Hog Island Bay, a shallow back-barrier lagoon on the Virginia coast where G. vermiculophylla makes up 85%–90% of the biomass. There was good agreement between laboratory and field measurements of N uptake and release. Daily N assimilation in field experiments (32.3±7.2 μmol N·g dw−1·d−1) was correlated with seasonal and local N availability. The average rate of N release across all sites and dates (65.8±11.6 μmol N·g dw−1·d−1) was 67% of gross daily uptake, and also varied among sites and seasons (range=33%–99%). Release was highest when growth rates and nutrient availability were low, possibly due to senescence during these periods. During summer biomass peaks, estimated N release from macroalgal mats was as high as 17 mmol N·m−2·d−1. Our results suggest that most estimates of macroalgal N uptake severely underestimate gross N uptake and that N is taken up, transformed, and released to the water column on short time scales (minutes–hours). [ABSTRACT FROM AUTHOR]

Marine invertebrate grazing on temperate macroalgae may exert a significant “top-down” control on macroalgal biomass. We conducted two laboratory experiments to test (1) if consumption by the omnivorous mud snail Ilyanassa obsoleta (Say) on the macroalga Ulva lactuca Linnaeus was a function of food quality (nitrogen content) and (2) if grazing on benthic macroalgae occurred at significant rates in the presence of alternative food sources in the sediment (detritus, larvae, benthic microalgae). Grazing rates were higher for N-enriched macroalgae; however, all snails lost weight when grazing on macroalgae alone, indicating that U. lactuca was a poor food source. The presence of sediment from two sites, a sandy lagoon and an adjacent organic-rich muddy tidal creek, did not affect consumption of macroalgae in microcosm experiments, and the grazing snails were capable of significantly reducing macroalgal biomass associated with both sediment types. Grazing rates by this omnivore were as high as 10.83 mg wet weight·individuals-1·d-1 and were similar to those recorded for herbivorous species. In situ loss rates calculated from average grazing rates per individual and snail abundances (up to 3.5 g dry weight·m-2·d-1) also were comparable with those calculated for herbivorous species. This level of grazing could remove up to 88% of new macroalgal growth at the lagoon site where the N supply was relatively low but had a much smaller effect (18% of new growth) at the high-nutrient creek site. Snails facilitated macroalgal growth at both sites by increasing tissue N content by 40%–80%. Consumption and digestion of macroalgae aided in the recycling of nutrients temporarily bound in the algae and resulted in enrichment of surficial sediments. Increased N sequestration in the sediments also was associated with an interruption of snail burrowing behavior due to persistent anoxia in sediments rich in decaying algal material. Our data suggest that in shallow lagoons where mud snails and benthic macroalgae coexist, grazing may influence N retention in macroalgal biomass. [ABSTRACT FROM AUTHOR]

Examines the influence of growth irradiance on the non-steady-state relationship between photosynthesis and tissue carbon and nitrogen (N) pools in Chaetomorpha linum (Chlorophyta) Kutzing in response to abrupt changes in external N availability. Acclimation of algae to low irradiance for a given thallus N content; Finding that N-saturated algae had the same photosynthesis-irradiance relationship regardless of light level.

Changes in the size of intracellular nitrogen pools and the potential feedback by these pools on etaximum N uptake (NH[SUB4] and NO[SUB3]) rates were determined for Chaetomorpha linum (Müller) Kützing grown sequentially under nutrient-saturating and nutrient-limiting conditions. The size of individual pools in N-sufficient could be ranked as residual organic N (RON) comprised mainly of amino acids and compounds > protein N > NO[SUB3] > NH[SUB4] chlorophyll N. When the external N supply was removed, growth rates remained high and individual N pools were depleted at exponential rates that reflected both dilution of existing pools by the addition of new biomass from growth and movement between the pools. Calculated fluxes between the tissue N pools showed that the protein pool increased throughout the N depletin period and thus did not serve a storage function. RON was the largest storage reserve; nitrate was the second largest, but more temporary, storage pool that was depleted within to days. Upon N resupply, the RON pool increased 3 × faster that either the inogrance or protein pools, suggesting that prtein synthesis was the rate-limiting step in N assimilation and caused a buildup of intermediate storage compuinds. Maximum uptake rates for both NH[SUB4] And NO[SUB3,SUP-] varied inversely with macroalagal N status and appeared to be controlled by changes in small intracellular N pools. Uptake of NO[SUB3,SUP-] showed an initial lag phase, but the initial uptake of NH[SUB4,SUP+] Pool was depleted in the absence of an external N supply. A strong negative correlation between the RON pool the and maximum assimilation uptake rates for both NH[SUB4,SUP+] And NO[SUB3,SUP-] suggested a feedback control or assimilatin uptake to the buildup and depletin of organic compounds. Enhanced uptake and the accumulatin of N as simple organic compounds or nitrate both provide a temporary mechanism to buffer against the asynchrony of N supply and demans in C. linum. [ABSTRACT FROM AUTHOR]

Routine determination of dissolved organic nitrogen (DON) is performed in numerous laboratories around the world using one of three families of methods: UV oxidation (UV), persulfate oxidation (PO), or high temperature combustion (HTC). Essentially all routine methods measure total dissolved nitrogen (TDN) and calculate DON by subtracting the dissolved inorganic nitrogen (DIN). While there is currently no strong suggestion that any of these methods is inadequate, there are continuing suspicions of slight inaccuracy by UV methods.This is a report of a broad community methods comparison where 29 sets (7 UV, 13 PO, and 9 HTC) of TDN analyses were performed on five samples with varying TDN and DIN concentrations. Analyses were done in a “blind” procedure with results sent to the first author. With editing out one set of extreme outliers (representing 5 out of 145 ampoules analyzed), the community comparability for analyzing the TDN samples was in the 8–28% range (coefficient of variation representing one standard deviation for the five individual samples by 28 analyses). When DIN concentrations were subtracted uniformly (single DIN value for each sample), the comparability was obviously worse (19–46% cv). This comparison represents a larger and more diverse set of analyses, but the overall comparability is only marginally better than that of the Seattle workshop of a decade ago. Grouping methods, little difference was seen other than inconclusive evidence that the UV methods gave TDN values for several of the samples higher than HTC methods. Since there was much scatter for each of the groups of methods and for all analyses when grouped, it is thought that more uniformity in procedures is probably needed. An important unplanned observation is that variability in DIN analyses (used in determining the final analyte in most UV and PO methods) is essentially as large as the variability in the TDN analyses.This exercise should not be viewed as a qualification exercise for the analysts, but should instead be considered a broad preliminary test of the comparison of the families of methods being used in various laboratories around the world. Based on many independent analyses here, none of the routinely used methods appears to be grossly inaccurate, thus, most routine TDN analyses being reported in the literature are apparently accurate. However, it is not reassuring that the ability of the international community to determine DON in deep oceanic waters continues to be poor. It is suggested that as an outgrowth of this paper, analysts using UV and PO methods experiment and look more carefully at the completeness of DIN conversion to the final analyte and also at the accuracy of their analysis of the final analyte. HTC methods appear to be relatively easy and convenient and have potential for routine adoption. Several of the authors of this paper are currently working together on an interlaboratory comparison on HTC methodology. [Copyright &y& Elsevier]