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Metaecosystem theory addresses the link between local (within habitats) and regional (between habitats) dynamics by simultaneously analyzing spatial community ecology and abiotic matter flow. Here we experimentally address how spatial resource gradients and connectivity affect resource use efficiency (RUE) and stoichiometry in marine phytoplankton as well as the community composition at local and regional scales. We created gradostat metaecosystems consisting of five linearly interconnected patches, which were arranged either in countercurrent gradients of nitrogen (N) and phosphorus (P) supply or with a uniform spatial distribution of nutrients and which had either low or high connectivity. Gradient metaecosystems were characterized by higher remaining N and P concentrations (and N∶P ratios) than uniform ones, a difference reduced by higher connectivity. The position of the patch in the gradient strongly constrained elemental stoichiometry, local biovolume production, and RUE. As expected, algal carbon (C)∶N, biovolume, and N-specific RUE decreased toward the N-rich end of the gradient metaecosystem, whereas the opposite was observed for most of the gradient for C∶P, N∶P, and P-specific RUE. However, at highest N∶P supply, unexpectedly low C∶P, N∶P, and P-specific RUE values were found, indicating that the low availability of P inhibited efficient use of N and biovolume production. Consequently, gradient metaecosystems had lower overall biovolume at the regional scale. Whereas treatment effects on local richness were weak, gradients were characterized by higher dissimilarity in species composition. Thus, the stoichiometry of resource supply and spatial connectivity between patches appeared as decisive elements constraining phytoplankton composition and functioning in metaecosystems.

Compared to phytoplankton, relatively little information exists about the importance of benthic algae as a source of dissolved organic C (DOC) in lakes. We enriched outdoor tanks with N and P in a full factorial design (unenriched, +N, +P, N+P) to evaluate effects of nutrients on the release, composition, and decomposition of DOC from Cladophora glomerata (L.) Kutz. in Lake Michigan. After 10 d of enrichment, biomass-specific rates of gross primary productivity (GPP) were significantly greater in the N+P treatment than in the +N or +P treatments. The fraction of fixed C released as DOC was ∼37% of GPP in the N+P treatment, which was similar to material collected from the lake (initial samples). Biomass-specific rates of DOC release did not vary significantly with treatment, whereas GPP declined in the nutrient-limited treatments (+N, +P, and unenriched). Consequently, DOC release represented a higher percentage of GPP (>100%) under severe nutrient limitation. Specific ultraviolet (UV) absorbance, ...

Biological N2 fixation hypothetically could balance the N∶P supply ratio in lakes and reservoirs, causing P to be the ultimate limiting element. However, the efficiency and time scale of this mechanism has been poorly studied and remains largely unknown. We compared the relative rates of N supply to the epilimnion of 3 reservoirs of similar age and morphology but with differing external nutrient inputs. Our objective was to determine if N2-fixation rates in these reservoirs exceeded other internal N supply pathways during the growing season, a situation that would suggest that these systems are still evolving in terms of nutrient-limitation status. Phytoplankton N2-fixation rates ranged from 3.4 to 16 g N m−2 y−1 in the reservoirs. These rates are among the highest N2-fixation rates reported in the scientific literature. Measureable N2 fixation also occurred in sediments in contact with reservoir epilimnia (0.85–1.5 g N m−2 y−1). Sediment N2-fixation rates exceeded rates of N regeneration from epi...

When individuals store resources acquired while moving through a spatially variable habitat, a form of population structure arises. The theoretical consequences of this process for resource competition are studied for phytoplankton species consuming a single nutrient resource, using a Lagrangian modeling approach. Each competitor population is divided into many subpopulations that move through two model habitats with gradients in nutrient availability: an unstirred chemostat and a partially mixed water column. The results provide little indication that resource storage contributes to competitive fitness in the scenarios analyzed. Superior competitors usually reduce the limiting nutrient to low concentrations at steady state or sometimes have high maximal growth rates. Resource storage enhances competitive fitness in temporally variable habitats where encounters with rich nutrient pulses are strongly periodic. However, in purely spatially variable habitats where encounters with rich nutrient patches are random, resource storage does not appear to provide much benefit, at least for passively moving organisms that cannot control their location.

Energy-based plant-herbivore models produce the "paradox of enrichment," a destabilizing influence of enrichment on population dynamics. Because many plants change their carbon : nutrient stoichiometry in response to the light : nutrient supply ratio, enrichment with light can cause a mismatch between the elemental compositions of plants and their herbivores. Herbivore growth rates may then decrease with increased light supply, which is termed the "paradox of energy enrichment." I present a stoichiometric phytoplankton-grazer model that accounts for the dynamical vertical light gradient and explore how algal and grazer densities, mineral nutrient concentration, algal nutrient stoichiometry, and system stability respond to enrichment with light (through changes in irradiance, background turbidity, and water column depth) versus enrichment with nutrients. Parameterized for Daphnia, the model produces several "unusual" phenomena: multiple equilibria (with grazers extinct in spite of high algal biomass at one equilibrium), inconsistent light enrichment effects on stability (light enrichment first destabilizes and then stabilizes), and the paradox of energy enrichment. These phenomena are restricted to the low end of realistic nutrient supplies except in very shallow systems, where high sedimentation rates effectively deplete the water column of nutrients. At higher nutrient supplies, light enrichment produces the classical paradox of enrichment, leading first to an increase in grazers at a stable equilibrium and then to algae-grazer oscillations.

Nutrient enrichment experiments were conducted in 1.2-m deep enclosures in 2 shallow, oligotrophic, mountain lakes. 15N-NO3 isotope tracer was used to compare the importance of phytoplankton and benthic compartments (epilithon, surface sediment [epipelon], and subsurface sediment) for NO3 uptake under high and low NO3 conditions. NO3 uptake approached saturation in the high-N lake, but not in the low-N lake. The capacity of phytoplankton and benthic compartments to take up NO3 differed among treatments and between lakes, and depended on water-column nutrient conditions and the history of NO3 availability. Phytoplankton productivity responded strongly to addition of limiting nutrients, and NO3 uptake was related to phytoplankton biomass and photosynthesis. However, more NO3 usually was taken up by benthic compartments (57–92% combined) than by phytoplankton, even though the response of benthic algal biomass to nutrient additions was less pronounced than that of phytoplankton and benthic NO3 uptake...

The Mackenzie Delta is a lake-rich region where the distribution and abundance of phytoplankton, macrophytes, and benthic algae among lakes may be controlled by an interaction between nutrient supply and sediment in the water column, and episodic reduction in light availability associated with river flooding. To simulate the effects of episodic river inflow on benthic algae, limnocorrals (depth = 3 m) were either not manipulated (control), or received high or low weekly additions of nutrients (N and P) and sediments, delivered as a pulse (1/wk: high-pulsed, low-pulsed) or distributed incrementally through each week (⅓ of high or low load × 3 d/wk: high-distributed, low-distributed) in a balanced triplicated design (15 limnocorrals). Artificial substrates, enriched to mimic the supply of nutrients at the sediment/water interface, were suspended in the limnocorrals at 2 m depth for 6 wk beginning in mid-July 1995. Maximum average levels of phytoplankton biomass (inferred from chlorophyll) occurred i...

Published data on stream periphyton, lake phytoplankton, and ocean phytoplankton were analyzed to 1) quantify regression models relating daily gross primary production (GPP) to chlorophyll a (chl a) standing stock and water temperature, 2) compare regressions across assemblages, and 3) compare the precision of regression estimates of daily primary production and of production integrated over time to those obtained by measurements using radioisotopes. Regression models predicting daily GPP explained between 29% and 86% of the variance in Log GPP with chl a accounting for 28 to 85% of the explained variance. Regression models differed significantly across assemblages. Chlorophyll-specific production, corrected for the effect of temperature, declined with increasing chl a standing stock presumably because of increased self shading, and was lower in stream periphyton than in lake or marine phytoplankton presumably because of reduced nutrient diffusion in algal mats. Gross primary production was more intensely...

Data from Lake Apopka, Florida, are presented to illustrate the importance of wind-induced resuspension of sediments in a shallow, subtropical, hypereutrophic lake. Wind-induced resuspension of meroplankton, primarily planktonic diatoms that settle to the benthic environment, accounts for a large component of the temporal variation in phytoplankton biomass. Chlorophyll concentrations >100 μg/L are highly correlated with wind speed, and the regression of wind speed on chlorophyll accounts for 53% of the temporal variability in chlorophyll. Resting cells of diatoms are also resuspended with the meroplanktonic community. Induction of resting cells that are physiologically dormant occurs in darkness; but within a few hours after exposure to the water-column light and nutrient environment, resting cells become physiologically active. Other types of resting propagules in addition to diatom resting cells also may be resuspended from sediments and affect phytoplankton dynamics. Data from Lake Apopka and other sys...

Freshwater crustacean zooplankton species show pronounced interspecific variability but approach intraspecific homeostasis in their stoichiometry. It is especially interesting that members of the dominating herbivores in freshwater systems, such as Daphnia species, show consistently high P:C ratios. Sestonic P:C ratios in epilimnetic waters are usually far below that encountered in the zooplankton, which suggests that grazers may approach a direct phosphorus limitation in many localities. This view was supported by a regional study of 47 lakes, in which most independent variables including phytoplankton biomass and chlorophyll were scarcely correlated with zooplankton biomass but particulate P (>0.45 μm) could explain a major part of the variance of zooplankton biomass. Of the various taxa, daphnids gave the best correlation with particulate P, cyclopids and rotifers the worst. These observations suggest that phytoplankton biomass per se may not be the major determinant of zooplankton biomass but that the...

GPP and chlorophyll a. Because tidal ranges reach, at most, 0.5 m in Waquoit Bay, and coverage of vegetation occurs only during a few days of spring tides each month, this mechanism does not seem convincing. More likely, denitrification and storage of watershed-derived nitrogen in salt marshes could be responsible for a sufficient reduction of nitrogen supplies to lower phytoplankton production. Similar results were obtained when we compared GPP or chlorophyll a in relation to the percent of the periphery of each estuary that is composed of salt marsh fringes (Fig. 1, bottom panels). We therefore conclude that the increase in producer activity due to nitrogen loading from watersheds may be mediated by the extent of salt marsh interposed between land and estuary. Further, salt marshes can decrease the nutrient supply to the estuaries and limit phytoplankton production and chlorophyll a levels. This work was supported by the WBMLER Research Experience for Undergraduates Grant and by a grant from NOAA Coastal Ocean Studies Program.

samples. The percentage of the total DOC pool utilized was similar among all three water samples: 15.7% in the forest, 10.3% in the agricultural, and 11 .O% in the urban samples. The organic matter from the agricultural watershed clearly had the greatest potential impact on the estuary. The unexpected conclusion from the experiment, however, is that the total quantity of organic matter is most important; the usable DOC made up a similar percentage of the total in water from all three ,60820 II/II/sIIIIIIIt 33-J watersheds. The research was supported by an LMER grant (NSF-OCE9214461). We thank lshi Buffam, David Giehtbrock, Charles Hopkinson, and Eileen Monaghan for advice and assistance with sample analysis.

A model of nutrient-limited phytoplankton growth, based on Michaelis-Menten uptake kinetics, is offered as an explanation of the so-called paradox of the plankton. The model suggests that an assemblage of coexisting phytoplankton may be limited by several nutrients, each species principally limited by the availability of a different nutrient. The principal requirement of the model is that each species be less effective at obtaining at low concentration the nutrient of which it requires larger amounts for continued growth. The model offers an explanation for the continued coexistence of associated phytoplankters in a stable equilibrium. It also offers a possible explanation for the decreased diversity commonly observed in eutrophic lakes and is consistent with observations that a larger number of different nutrients are in short supply in oligotrophic waters.

Observations at near surface levels in the Gulf of Maine in September of 1966 show that phytoplankton species coexist in equilibrium with any losses through sinking balanced by growth through sharing nutrient. This hypothetical equilibrium is justified, and brought clearly into focus, by observations at deeper levels, where just the opposite occurs: lack of equilibrium and lack of balance between cell loss and nutrient gain. Comparison of observation and a mathematical model makes clear an unbridgeable distinction. What you can know so clearly from the model, that under equilibrium conditions coexisting species are prevented from not existing, you can never know from observing. All you can know is that they exist, when you observe. Moreover, it would be wrong to say that the species that are not observed in September do not exist or are excluded, when these species always dominate in spring. For such species are potentially present when they are not actually present, in September. Otherwise there would be...

A mechanistic, size-structured model was used to analyze effects of grazers on lacustrine primary productivity. Dependence of grazing and nutrient excretion rates on herbivore size, and dependence of algal metabolic rates on cell size, interacted to produce strong responses in primary productivity. Productivity was maximum at intermediate concentrations of chlorophyll and zooplankton biomass. This unimodal response is consistent with data from aquatic systems, terrestrial systems (where the underlying mechanisms are different), and theoretical expectations. Substantial variability in lake productivity (2 to 3 orders of magnitude) exists that cannot be explained by nutrient supply. Food web interactions can regulate herbivorous zooplankton size and abundance, independent of nutrient supply. The resultant variability in herbivory influences nutrient recycling and the size distribution of the phytoplankton, and can alter lake primary productivity by as much as two orders of magnitude.

Different clones of phytoplankton isolated from the Sargasso Sea and from temperate-zone estuaries were grown in culture and exposed to several toxic chemicals which, presumably, they have never experienced in their evolutionary histories. Intraspecific differences in sensitivity were noted; the growth of the estuarine clones was always less affected by these novel chemical stresses than was the growth of the oceanic clones. The data indicate that phytoplankton and other organisms which have evolved in and adapted to physically variable environments would, because of their adaptations, be better able to tolerate any toxic compound (and possibly any perturbation) than would morphologically similar organisms adapted to stable environments. Further experimental evidence suggests the degree of sensitivity of phytoplankton to pollutants may be correlated with certain adaptations of their membrane systems; open-ocean cells, whose membrane systems permit nutrient exploitation and maximal growth at low ambient co...

Experiments were carried out to determine if photosynthetic rates of phytoplankton were nutrient-limited in a temperate lake and in an arctic pond. They were also designed to determine if daily or seasonal patterns existed.Photosynthetic rates were measured under defined light intensities and temperatures, at various concentrations of sodium phosphate. Daily patterns were determined with 5 or 7 measurements made over a 24 to 30 hour interval.Photosynthetic rates were stimulated by the addition of phosphate to samples of water from Lake George, New York. They were stimulated on only some of the sampling dates during one year of survey. When phosphate was not limiting, neither were other essential nutrients, with one exception. Later work, in the arctic, suggested that a negative result should not be interpreted as a non-limiting situation, however.Arctic phytoplankton show strong daily oscillations in photosynthetic rates. The daily maximum occurred most frequently in the early evening, at 1800; that is, a...

A three-compartment mathematical model was developed to represent a phytoplankton population with the capability of storing nitrogen in a nitrate-limited environment. Parameters were estimated by fitting the model to equilibrium data from two chemostat experiments reported by Caperon (1968, 1969). The model was then run to simulate the transient chemostat conditions, and the model response was compared with the observed data. The model provides a reasonable representation of the sudden population surges associated with intracellular nutrient storage. One of the most interesting phenomena associated with phytoplankton communities is the ability for many different genera and species to coexist in the same apparently homogeneous environment. Because of the sequential blooms observed in nature, it is intuitively obvious that temporal variations in environmental conditions are a predominant mechanism influencing an organism's ability to compete and coexist with its competitors. However, in the past, true coexi...

Cell division in batch cultures of the tropical oceanic diatom, Chaetoceros gracilis, was measured in artificial medium containing different initial amounts of phosphate. The rate of growth was limited by phosphate concentrations below approximately 0.22 µg.at./l. and appeared to be a linear function of concentration rather than a hyperbolic function. Ks in the hyperbolic relationship equating growth rate and concentration would be about 0.12 µg.-at./l. The results substantiate an earlier suggestion that low phosphate did not limit the rate of phytoplankton growth in the northeastern tropical Pacific Ocean; and they also verify previous use of a linear nutrient relationship in productivity equations given by other authors. Pauses in cell division at low phosphate concentrations are either attributed to the induction of phosphatases which can act on stored intracellular phosphorus or to low extracellular concentrations. Final cell numbers were a linear function of initial phosphate concentration up to abou...