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Mountain glaciers are retreating rapidly due to climate change, leading to the formation of downstream lakes. However, little is known about the physical and biogeochemical conditions in these lakes across a range of glacial influence. We surveyed alpine lakes fed by both glacial and snowpack meltwaters and those fed by snowpack alone to compare nutrient concentrations, stoichiometry, water clarity, chlorophyll, and zooplankton communities. Total phosphorus (TP) and soluble reactive phosphorus were two times higher in glacial lakes than in non‐glacial lakes, while nitrate concentrations were three times higher. However, organic carbon concentrations in glacial lakes were two times lower than in non‐glacial lakes. The carbon‐to‐phosphorus ratio and the nitrogen‐to‐phosphorus ratio of lake seston increased with water clarity in glacial lakes, suggesting that turbidity from glacial flour increases light limitation and increases stoichiometric food quality for zooplankton in newly formed lakes. However, chlorophyll aconcentrations did not differ between lake types. Through structural equation modeling, we found that glaciers exhibit a bidirectional association with nitrate and TP concentrations, perhaps mediated through landscape vegetation and lake clarity. Zooplankton communities in high‐turbidity glacial lakes were largely composed of cyclopoid copepods and rotifers (i.e., non‐filter feeders), while non‐glacial lakes tended to be dominated by calanoid copepods and cladocerans (i.e., filter feeders). Our results show that glacier‐influenced lakes have biogeochemical and ecological characteristics distinct from snow‐fed mountain lakes. Sustained studies are needed to assess the dynamics of these unique features as the influence of the alpine cryosphere fades under ongoing climate change.

The article presents an interview with James Elser, Bierman Professor of Ecology at the University of Montana (UM) and Director of UM's Flathead Lake Biological Station at Yellow Bay. He discusses the importance of ecological stoichiometry, highlighting its application in diverse habitats including Flathead Lake, mountain lakes in western Montana, and ecosystems in western China.

Eutrophication mitigation is an ongoing priority for aquatic ecosystems. However, the current eutrophication control strategies (phosphorus (P) and/or nitrogen (N)) are guided mainly by nutrient addition experiments in small waters without encompassing all in-lake biogeochemical processes that are associated largely with lake morphological characteristics. Here, we use a global lake data set (573 lakes) to show that the relative roles of N vs P in affecting eutrophication are underpinned by water depth. Mean depth and maximum depth relative to mixing depth were used to distinguish shallow (mixing depth > maximum depth), deep (mixing depth < mean depth), and transitional (mean depth ≤ mixing depth ≤ maximum depth) lakes in this study. TN/TP ratio (by mass) was used as an indicator of potential lake nutrient limitation, i.e., N only limitation if N/P < 9, N + P colimitation if 9 ≤ N/P < 22.6, and P only limitation if N/P ≥ 22.6. The results show that eutrophication is favored in shallow lakes, frequently (66.2%) with N limitation while P limitation predominated (94.4%) in most lakes but especially in deep ones. The importance of N limitation increases but P limitation decreases with lake trophic status while N and P colimitation occurs primarily (59.4%) in eutrophic lakes. These results demonstrate that phosphorus reduction can mitigate eutrophication in most large lakes but a dual N and P reduction may be needed in eutrophic lakes, especially in shallow ones (or bays). Our analysis helps clarify the long debate over whether N, P, or both control primary production. While these results imply that more resources be invested in nitrogen management, given the high costs of nitrogen pollution reduction, more comprehensive results from carefully designed experiments at different scales are needed to further verify this modification of the existing eutrophication mitigation paradigm.

AbstractNumerous studies in ecological stoichiometry have assessed responses of Daphniato single- and multi-species food resources of varying nutritional quality. Diets based on P-limited algae with a high biomass carbon to phosphorus (C:P) ratio impose decreased growth on Daphniabecause P demands for growth are not met. More recently, laboratory studies have shown that P-rich algal food also results in decreased growth rates for Daphnia; however, no published study has tested the impact of low food C:P on Daphniaunder field conditions. To address this, we measured growth rate as well as respiration and ingestion rates for D. magna, D. pulicaria, and D. pulexthat were fed lake seston experimentally enriched with phosphate (PO43−). Lake seston during the experimental period successfully took up PO43−enrichment treatments and was dominated by cyanobacteria and haptophytes, primarily those from the genus Prymnesium. Growth rate reductions for D. magnawere strong in response to high-P food, most likely as a result of decreased ingestion rate; however, growth rate responses for D. pulexand D. pulicariawere modest and not statistically significant, although significant responses in respiration rates were observed for all species. Our study extends laboratory findings that P-rich food can have deleterious impacts on Daphniato field conditions. We also found diverse responses among species, however, possibly caused by variations among taxa in body stoichiometry or physiology, differences in ambient and treatment seston C:P for the various experiments, or the effects of diet diversity under high-P conditions.

Eutrophication is the most widespread water quality issue globally. To date, most efforts to control eutrophication have focused on reductions of external nutrient inputs, yet importance of nutrient stoichiometry and subsequent shift in plankton composition in aquatic ecosystem has been largely neglected. To address eutrophication, improved sanitation is one of the United Nations Sustainable Development Goals, spurring the constructions of wastewater treatment facilities that have improved water quality in many lakes and rivers. However, control measures are often targeted at and effective in removing a single nutrient from sewage and thus are less effective in removing the others, resulting in the changes of nutrient stoichiometry. In general, more effective phosphorus removal relative to nitrogen has occurred in wastewater treatment leading to substantial increases in N/P ratios in effluent relative to the influent. Unfortunately, high N/P ratios in receiving waters can impose negative influences on ecosystems. Thus, long-term strategies for domestic wastewater management should not merely focus on the total reduction of nutrient discharge but also consider their stoichiometric balances in receiving waters.

Global food production depends on phosphorus. Phosphorus is broadly applied as fertilizer, but excess phosphorus contributes to eutrophication of surface water bodies and coastal ecosystems. Here we present an analysis of phosphorus fluxes in three large river basins, including published data on fertilizer, harvested crops, sewage, food waste and river fluxes. Our analyses reveal that the magnitude of phosphorus accumulation has varied greatly over the past 30–70 years in mixed agricultural–urban landscapes of the Thames Basin, UK, the Yangtze Basin, China, and the rural Maumee Basin, USA. Fluxes of phosphorus in fertilizer, harvested crops, food waste and sewage dominate over the river fluxes. Since the late 1990s, net exports from the Thames and Maumee Basins have exceeded inputs, suggesting net mobilization of the phosphorus pool accumulated in earlier decades. In contrast, the Yangtze Basin has consistently accumulated phosphorus since 1980. Infrastructure modifications such as sewage treatment and dams may explain more recent declines in total phosphorus fluxes from the Thames and Yangtze Rivers. We conclude that human-dominated river basins may undergo a prolonged but finite accumulation phase when phosphorus inputs exceed agricultural demand, and this accumulated phosphorus may continue to mobilize long after inputs decline.

AbstractRecently, data have emerged indicating that not only high food carbon:phosphorus (C:P) ratio but also low food C:P (P-rich food) can have negative effects on the growth of consumers. The shape of this “stoichiometric knife edge,” however, is not yet well-documented, and the mechanisms underpinning it are not understood. Here we report the results of experiments using 3 species of Daphnia(D. magna, D. pulicaria, D. pulex) consuming the green alga Scenedesmus acutuswith widely varying C:P ratios (from <50 to >1500 by atoms). The experiments were designed to (1) characterize the potential stoichiometric knife edge for each species, and (2) evaluate potential changes in feeding and respiration rates that may underpin the unimodal response to food C:P. All 3 Daphniaspecies grew more slowly when food C:P (atomic) exceeded ~250–300 but also when C:P was <120. Both high and low C:P foods were associated with increased respiration rates, indicating that the negative effects of food C:P imbalance at least partially involve increased metabolic costs of dealing with stoichiometrically imbalanced food. Feeding rate experiments indicated that, in contrast with limited previous data, animals generally increased their feeding rate on P-rich food. Overall, the “lower threshold elemental ratio” we identify here (~120) is surprisingly high, in an ecologically meaningful range, suggesting that negative effects of excessive food P content may play an under-recognized role in affecting Daphniaperformance in P-rich lakes with low seston C:P ratio. Such effects also need to be incorporated into stoichiometrically explicit models of planktonic trophic interactions.

AbstractManaging lake eutrophication requires a clear understanding of resource limitation of primary productivity, yet historically research on this subject has focused on temperate lakes. In 2010, we quantified several metrics of resource limitation in Lake Atitlán, Guatemala, Central America’s deepest tropical mountain lake that has recently experienced extensive phytoplankton blooms. In contrast to many temperate lakes, Lake Atitlán did not show a relationship between total phosphorus (TP) and chlorophyll a (Chl-a) concentrations. Average molar ratios of total nitrogen (TN) to TP decreased from 16.4 to 4.5 between stratified and mixing conditions. During our monitoring period, Tropical Storm Agatha landed on Guatemala, washing in sediment from the watershed, and concentrations of P temporarily increased in the lake by >60%. Initial experimental bioassays indicated phytoplankton growth was colimited by N and P prior to the storm, whereas post-storm assays suggested limitation by P and trace elements. Compared to previous years, Limnoraphis robusta, an N-fixing cyanobacterium, increased in abundance earlier in the year; however, there was no bloom event during our sampling period. Experimental studies indicated that nitrogenase activity was limited by P and iron availability while light was secondarily limiting of overall phytoplankton growth. This study illustrates the potential for baseline nutrient and phytoplankton growth dynamics to substantially differ for tropical lake systems from conventionally studied temperate lakes and the potential for “pulse” events (e.g., tropical storms) to alter those dynamics.

Astrobiology is an extremely wide-ranging field and thus is in special need of conceptual and theoretical frameworks that can integrate its various arenas of study. In this paper I review recent work associated with a conceptual framework known as ‘ecological stoichiometry’ and even more recent extensions in the development of ‘biological stoichiometry’. Ecological stoichiometry is the study of the balance of energy and multiple chemical elements in ecological interactions and has developed rapidly in the study of nutrient cycling and energy flow in aquatic food webs. It identifies the elemental composition of interacting biota as central in understanding the nature of their interactions and dynamics, including key feedbacks via nutrient recycling. Biological stoichiometry extends this mode of thinking to all types of biological systems. It especially seeks to better understand, at the biochemical and genetic levels, the factors influencing the elemental composition of living things and the evolutionary forces that drive and constrain that elemental composition. By connecting key concepts of ecosystem ecology, evolutionary biology and biochemistry, stoichiometric theory integrates biological information into a more coherent whole that holds considerable promise for application in astrobiology. Several examples of potential astrobiological applications of stoichiometric analysis are offered, including ones related to pre-biotic evolution, the Cambrian explosion, biosignatures and biological feedbacks on planetary carbon cycling.

The P, N, and C content of littoral macroinvertebrates from 8 lakes located in 3 distinct geographical regions of Canada (central Alberta, northwestern Ontario, and Rocky Mountains near Jasper, Alberta) is described. A wide range of values was found in the body content (all values are % of dry mass) of P (0.4-1.6%), N (5.8-13.7%), and C (32.5-53.5%) in the data set containing invertebrates from all 8 lakes. C:P (63-324), N:P (9.6-60), and C:N (4.2-7.6) (all by atom) also varied widely. This variation was partly related to the different mean body content of P, N, and C among taxonomic groups. However, the mean P, N, and C content of macroinvertebrate assemblages varied little among lakes. The patterns of elemental composition in benthic invertebrates shown here are similar to zoo-plankton and terrestrial insects, and indicate that the strength of stoichiometric constraints acting in littoral food webs will depend on the taxa being considered.

It has recently been hypothesized that variation in the intergenic spacer (IGS) of rDNA has considerable developmental, evolutionary and ecological significance through effects on growth rate and body C : N : P stoichiometry resulting from the role of the IGS in production of rRNA. To test these ideas, we assessed changes in size and structure of the repetitive region of the IGS, juvenile growth rate (JGR), RNA and phosphorus (P) contents in clonal lineages of Daphnia pulexderived from a single female and subjected to divergent selection on weight–specific fecundity (WSF). As a result of selection, WSF diverged rapidly, with significant reductions within two generations. Other significant changes accompanying shifts in WSF were that juveniles produced by low–WSF females grew more rapidly and had higher RNA and P contents. An increased predominance of long IGS variants was observed in lineages with elevated JGRs and low WSF. The observed variations in IGS length were related to the number of subrepeat units carrying a promoter sequence in the repetitive region. These results strongly support the hypothesized relationships, indicate a genetic mechanism for the evolution of such associations and demonstrate that Daphnia(and perhaps other parthenogens) possess considerable potential for rapid adaptive change in major life–history traits.

We examined how the intensity of benthic grazing affects the C:P stoichiometry of epilithon in an oligotrophic lake at the Experimental Lakes Area in northwestern Ontario, Canada. Caddisflies were enclosed over natural epilithon to compare epilithic C:P ratios along a grazer gradient (0, 1, 4, and 9 individuals/700 cm2 enclosure) after 10 and 20 d. We also examined mechanisms that potentially explain changes in epilithic C:P ratios at different grazer levels by estimating algal biomass and taxonomic composition. After 10 d, no differences in epilithic C:P ratios were found among grazer levels. However, after 20 d, epilithic C:P ratios (~500) at high grazer levels (9/enclosure) were significantly lower than C:P ratios (~1200) in grazer-free enclosures. We also found that algal biomass was significantly reduced under intense grazing. No significant effects of grazer level were detected on epilithic C:chlorophyll ratios, the % contribution of algal C to epilithic organic C, or the taxonomic composition of algal communities after 10 or 20 d. In addition, only ~2 to 3% of epilithic C was estimated to be in benthic algae throughout the experiment. These results indicate that differences in epilithic C:P ratios caused by increased grazing intensity cannot be explained by changes in prevalence of different organic C pools (detritus and algae) comprising the epilithon, changes in the nutrient content of algae, or by the taxonomic composition of the algal community. Factors affecting the nutrient content of detritus likely account for the lower epilithic C:P ratios found in highly grazed enclosures.

Abstract: All organisms are composed of multiple chemical elements such as carbon, nitrogen and phosphorus. While energy flow and element cycling are two fundamental and unifying principles in ecosystem theory, population models usually ignore the latter. Such models implicitly assume chemical homogeneity of all trophic levels by concentrating on a single constituent, generally an equivalent of energy. In this paper, we examine ramifications of an explicit assumption that both producer and grazer are composed of two essential elements: carbon and phosphorous. Using stoichiometric principles, we construct a two-dimensional Lotka-Volterra type model that incorporates chemical heterogeneity of the first two trophic levels of a food chain. The analysis shows that indirect competition between two populations for phosphorus can shift predator—prey interactions from a (+, −) type to an unusual (−, −) class. This leads to complex dynamics with multiple positive equilibria, where bistability and deterministic extinction of the grazer are possible. We derive simple graphical tests for the local stability of all equilibria and show that system dynamics are confined to a bounded region. Numerical simulations supported by qualitative analysis reveal that Rosenzweig’s paradox of enrichment holds only in the part of the phase plane where the grazer is energy limited; a new phenomenon, the paradox of energy enrichment, arises in the other part, where the grazer is phosphorus limited. A bifurcation diagram shows that energy enrichment of producer—grazer systems differs radically from nutrient enrichment. Hence, expressing producer—grazer interactions in stoichiometrically realistic terms reveals qualitatively new dynamical behavior.

All organisms are composed of multiple chemical elements such as carbon, nitrogen and phosphorus. Whileenergy flowand element cyclingare two fundamental and unifying principles in ecosystem theory, population models usually ignore the latter. Such models implicitly assume chemical homogeneity of all trophic levels by concentrating on a single constituent, generally an equivalent of energy. In this paper, we examine ramifications of an explicit assumption that both producer and grazer are composed of two essential elements: carbon and phosphorous. Using stoichiometric principles, we construct a two-dimensional Lotka–Volterra type model that incorporates chemical heterogeneity of the first two trophic levels of a food chain. The analysis shows that indirect competition between two populations for phosphorus can shift predator–prey interactions from a (+, −) type to an unusual (−, −) class. This leads to complex dynamics with multiple positive equilibria, where bistability and deterministic extinction of the grazer are possible. We derive simple graphical tests for the local stability of all equilibria and show that system dynamics are confined to a bounded region. Numerical simulations supported by qualitative analysis reveal that Rosenzweig’s paradox of enrichment holds only in the part of the phase plane where the grazer is energy limited; a new phenomenon, the paradox of energy enrichment, arises in the other part, where the grazer is phosphorus limited. A bifurcation diagram shows that energy enrichment of producer–grazer systems differs radically from nutrient enrichment. Hence, expressing producer–grazer interactions in stoichiometrically realistic terms reveals qualitatively new dynamical behavior.

Abstract: Changes in the ecological stoichiometry of C, N, and P in the pelagic zone are reported from a whole-lake manipulation of the food web of Lake 227, an experimentally eutrophied lake at the Experimental Lakes Area, Canada. Addition of northern pike eliminated populations of planktivorous minnows by the third year (1995) after pike introduction, and in the fourth year after pike addition (1996), a massive increase in the abundance of the large-bodied cladoceran Daphnia pulicaria occurred. Accompanying this increase in Daphnia abundance, zooplankton community N:P declined, seston concentration and C:P ratio decreased, and dissolved N and P pools increased. During peak abundance, zooplankton biomass comprised a significant proportion of total epilimnetic phosphorus (greater than 30%). During the period of increased Daphnia abundance, concentrations of dissolved inorganic nitrogen (TIN) increased more strongly than dissolved phosphorus (TDP), and thus TIN:TDP ratios were elevated. Sedimentation data indicated that increased grazing led to greatly reduced residence times of C, N, and especially P in the water column during 1996. Finally, previously dominant N-fixing cyanobacteria were absent during 1996. Our results show that strong effects of food-web structure can occur in eutrophic lakes and that stoichiometric mechanisms play a potentially important role in generating these effects.

Lake Dianchi is one of the most phosphorus-polluted lakes in China. Understanding its phosphorus budget and the corresponding limitations for research are fundamental to lake management. We have therefore systematically analyzed publications and publicly available datasets regarding Dianchi P pollution since the year 1960 to explore the anthropogenic perturbation on phosphorus budget dynamics. Results show that current exogenous loading (> 600 tons ⋅a−1) remains significant; inflow rivers and drainage pipes around the lakeshore are the main pathways (accounting for around 80% of loading). Besides shifts in the spatial–temporal pattern of exogenous loading, artificial treatments have also reshaped P removal patterns; consequently, only a small amount of P can be removed through lake outflows (<20%, around 140 tons ⋅a−1). Although a reduction has been observed in exogenous P loading over the past decade, endogenous P loading has significantly increased, of which P flux via sediment release alone approximates more than 180 tons ⋅a−1in 2017. Generally, P budget improvements have been due to artificial treatments such as the Niulan River water diversion project, shifting directions on tail water emissions, and sediment dredging projects. However, these treatments also introduce negative consequences and therefore require further caution. The reuse of ”waste P” via treatments has especially become a challenge. To date, exogenous loading via atmospheric dry deposition, wintering birds, water diversion, degradation in geological phosphorus-rich regions and endogenous P cycling fluxes, etc., need further study. Here, suggestions for corresponding research are given. It is suggested that the restoration of spontaneous ”P balance” in the Dianchi system, and the modification of the regional urban development strategy should present issues facing plateau freshwater lakes in SW China which need to be addressed. Moreover, synthesized analyses and machine learning processes are new attempts worth promoting to other lakes where monitoring data is incomplete.

The ability of nutrient enrichment bioassays versus physiological indicators to detect ecosystem changes in nutrient cycling were compared. Field data spanned four years in three lakes, two of which were subjected to fish manipulations in year two. The manipulations changed phytoplankton and zooplankton assemblages and nutrient cycling. Phosphorus and nitrogen enrichments (phosphorus enrichment bioassay (PEB), nitrogen enrichment bioassay, nitrogen plus phosphorus enrichment bioassay, specific alkaline phosphatase activity, ammonium enhancement response, and total nutrients (total phosphorus, total nitrogen) were measured before and after the manipulations. Although physiological indicators were often more sensitive, nutrient enrichments and physiological indicators both led to the same conclusions regarding changes in nutrient cycling. Total nutrients did not respond to manipulation. Physiological indicators measure the extant algal assemblage, are rapidly assayed, can be run more frequently than nutrient enrichments, and lead to the same conclusions as nutrient enrichment bioassays.

1. Species compositions of zooplankton and phytoplankton were followed in Tuesday Lake before and after experimental manipulation of its fish populations (addition of piscivorous largemouth bass, removal of planktivorous minnows). Plankton dynamics were compared to those of adjacent, unmanipulated Paul Lake, where piscivorous fish have been dominant historically. 2. Indices of similarity for the zooplankton communities in the two lakes in 1984 prior to the manipulation were low; however, following the manipulation in spring, 1985, similarity of the zooplankton in the two lakes rose considerably and remained high throughout 1986. This was the result of an increase in Tuesday Lake of previously rare large-bodied cladocerans (Daphnia pulex, Holopedium gibberum) which were the dominants in Paul Lake, and the disappearance in Tuesday Lake of the dominant small-bodied copepod Tropocyclops prasinus, a minor component of the Paul Lake zooplankton. These observations are consistent with prior observations of the effects of size-selective predation on zooplankton communities. 3. Phytoplankton communities also responded strongly to the manipulation, with similarity indices for the two lakes rising from low levels in 1984 to high levels of similarity in 1985 and 1986, reflecting the decrease of formerly dominant Tuesday Lake taxa which were unimportant in Paul Lake and the appearance or increase in Tuesday Lake of several taxa characteristic of the Paul Lake phytoplankton assemblage. these results clearly show that food web structure can have pronounced effects on community composition at all levels of the food web, and that, just as zooplankton communities are structured by sizeselective predation, phytoplankton communities are structured by herbivory. These observations may provide some insight into factors governing the complex distributions of phytoplankton species among various lakes.

Abstract: We report here the results of an experimental study designed to compare algal responses to short-term manipulations of zooplankton in three California lakes which encompass a broad range of productivity (ultra-oligotrophic Lake Tahoe, mesotrophic Castle Lake, and strongly eutrophic Clear Lake). To assess the potential strength of grazing in each lake, we evaluated algal responses to a 16-fold range of zooplankton biomass. To better compare algal responses among lakes, we determined algal responses to grazing by a common grazer (Daphnia sp.) over a range ofDaphnia densities from 1 to 16 animals per liter. Effects of both ambient grazers andDaphnia were strong in Castle Lake. However, neither ambient zooplankton norDaphnia had much impact on phytoplankton in Clear Lake. In Lake Tahoe, no grazing impacts could be demonstrated for the ambient zooplankton butDaphnia grazing had dramatic effects. These results indicate weak coupling between phytoplankton and zooplankton in Clear Lake and Lake Tahoe, two lakes which lie near opposite extremes of lake trophic status for most lakes. These observations, along with work reported by other researchers, suggest that linkages between zooplankton and phytoplankton may be weak in lakes with either extremely low or high productivity. Biomanipulation approaches to recover hypereutrophic lakes which aim only to alter zooplankton size structure may be less effective if algal communities are dominated by large, inedible phytoplankton taxa.

We compared the results of phosphorus-enrichment bioassay experiments with alkaline phosphatase activity (APA) assays as indicators of phosphorus (P) limitation of in situ phytoplankton growth. In 4-d experiments, phytoplankton APA decreased or remained unchanged in P-enriched samples, but increased in unenriched samples, indicating a rapid alteration of the P status of the unenriched algae during the experimental incubations. In direct comparisons of enrichment bioassays and APA assays of reservoir phytoplankton samples, the results of the two methods corresponded in general, although contradictory results were not uncommon. Our data support the conclusion that enrichment experiments can indicate the potential for nutrient limitation of algal growth in the absence of other limiting factors, but do not necessarily demonstrate the occurrence of in situ nutrient limitation of phytoplankton production.

Mesocosm experiments were performed in three north temperate lakes to evaluate responses of algal chlorophyll, primary productivity (PPR), and chlorophyll-specific PPR (photosynthetic capacity) to experimental manipulations of zooplankton biomass and size distribution. In all experiments, nutrient enrichment led to increased growth and higher photosynthetic capacity, indicating that the phytoplankton in these lakes were nutrient-limited. In experiments which exposed phytoplankton to a 32-fold range of zooplankton biomass, chlorophyll decreased significantly (p < 0.05), while PPR was unchanged. As a result, specific PPR increased significantly with increasing zooplankton biomass, indicating an improvement in algal physiological status and the occurrence of compensatory growth in response to increased grazing pressure and higher nutrient regeneration. In an experiment which manipulated the zooplankton size distribution at two constant levels of zooplankton biomass (¼ x and 2 x ambient zooplankton density), no significant algal responses to zooplankter size occurred at the low biomass level; however, at the high level, chlorophyll decreased and specific PPR increased with increasing average zooplankter size. These results emphasize the close coupling between zooplankton grazing and nutrient regeneration and algal nutrient uptake and growth in such nutrient-deficient systems.

AbstractStoichiometric theory predicts that zooplankton with low body N:P ratio recycle nutrients at higher N:P ratios than zooplankton taxa with low body N:P ratio, an effect that may accentuate P-limitation of phytoplankton growth. However, existing theory assumes that all regenerated nutrients are returned to the dissolved pool and does not account for the counteracting possibility that materials not assimilated by zooplankton might be lost from the water column via sedimentation. To assess the stoichiometry of zooplankton effects on N and P sedimentation, a survey of sedimentation, zooplankton, and suspended particulate matter was conducted in twelve lakes in the vicinity of the Experimental Lakes Area, Canada. Concentrations and ratios of carbon, nitrogen, and phosphorus in suspended particulate matter in the mixed layer of each lake were determined. Zooplankton community composition, biomass, and elemental composition (N:P ratio) for each lake were assessed. Finally, sediment traps were deployed in the hypolimnion of each lake to quantify rates of particulate matter sedimentation as well as C:N, C:P, and N:P ratios of sedimenting material. C:P and N:P ratios of sedimented material were considerably lower than those in seston but C:N ratios were similar. The N:P ratio of sedimented material had no relationship with seston N:P but decreased strongly with zooplankton N:P, consistent with stoichiometric recycling theory. However, the residuals of sedimented N:P versuszooplankton N:P relationship had a negative relationship with seston N:P, suggesting a joint stoichiometric dependence of sedimentation on consumer and seston elemental composition with consumer N:P ratio of primary importance. These patterns suggest that water column processes integrate the stoichiometry of microbial processing of detrital materials with that of consumer-driven nutrient cycling to determine sediment elemental composition.

The effect of cell size on chlorophyll-specific light extinction (the “package effect”) was investigated by examining the size distribution of algal chlorophyll relative to light extinction both in a broad survey of twenty lakes with differing concentrations of dissolved color and chlorophyll and in an intensive study of two, relatively low-color lakes (Paul and Peter lakes). In the lakes sampled in the lake survey, water color was the dominant factor influencing light penetration. This factor, along with a significant correlation between water color and chlorophyll concentration, made the effects of algal size on light extinction difficult to evaluate. No evidence for the operation of the package effect was found in these data. In Paul and Peter lakes, chlorophyll-specific light extinction was higher when nanoplankton dominated the algal community. This result suggests that, in relatively uncolored lakes and in the ocean, changes the algal size distribution will have significant effects on the penetration of light in the water column.

ABSTRACT: A whole-lake manipulation of food-web structure (introduction of a top predator, northern pike, to a minnow-dominated lake) was performed in a Canadian Shield lake (L110) to examine the stoichiometric consequences of changes in planktonic community structure generated by altered food-web structure. Minnow abundance, zooplankton biomass and community composition, microconsumer abundance, and concentration and carbon–phosphorus (C:P) ratio of suspended particulate matter were monitored in L110 and unmanipulated L240 before (1992) and after (1993–95) pike introduction. Algal biomass in L110 determined from microscopic examination for postmanipulation and premanipulation periods was also compared with dynamics in a suite of unmanipulated reference lakes from long-term monitoring records. Pike were added in spring in 1993 and 1994 in sufficient quantity to raise pike biomass to levels of around 22 kg ha−1 by 1994. Minnow populations in L110 responded dramatically, decreasing to levels 30% (1993), 10% (1994), and less than 1% (1995) of premanipulation values. However, most components lower in the food web did not respond in a manner consistent with predictions of existing food-web theory, such as the idea of cascading trophic interactions (CTI). While Daphnia biomass increased in L110 in the first year following manipulation, consistent with CTI, this effect was temporary and Daphnia collapsed in 1995, the year of lowest minnow abundance. Total zooplankton biomass in both lakes declined during the study period and, contrary to CTI, this decline appeared somewhat stronger in L110 than in L240. Dominant microconsumers (heterotrophic microflagellates) did not differ among years in either lake and did not appear to respond to food-web manipulation. At the bottom of the food web, no changes in bacterial biomass occurred in either lake. However, total concentrations of particulate matter appeared to increase in L110 after manipulation (contrary to expectations based on the theory of CTI) while algal biomass did not change in the manipulated lake relative to reference systems. Finally, particulate C:P increased in both L110 and L240 during the study period. The lack of strong response of Daphnia, the lack of response of the microbial food web, decreases in zooplankton biomass and increases in particulate biomass following reduction of minnow populations after piscivore introduction are at odds with expectations from existing food-web theory, such as the idea of CTI as currently formulated. However, the extremely high C:P ratios in particulate matter at the base of the food webs in these lakes, the coincidence of zooplankton declines and increases in particulate C:P ratios, and the results of small-scale mesocosm food-quality experiments are consistent with a hypothesis of a stoichiometric constraint operating on food-web dynamics in this and similar ecosystems.

A series of eight nutrient‐deletion/dilution‐gradient experiments was performed in Castle Lake during summer 1993 to quantify and characterize microconsumer grazing and contributions of nutrient supply sources (external, cell quota, recycling) supporting phytoplankton production. Responses of net chlorophyll production rate to dilution under nutrient‐saturated conditions were frequently nonlinear, indicating saturation of micrograzer feeding at low biomass levels within the dilution gradient (dilutions of < 10–30% whole lake water). Despite feeding saturation, micrograzers exerted substantial grazing pressure on phytoplankton: microconsumer grazing coefficients (0.05–0.22 d–1, mean: 0.14 d−1) exceeded previous measures of crustacean grazing in this system. Nonlinear feeding kinetics required that piecewise multiple regression be used to estimate contributions of external, cell quota, and recycling to N and P supply. In deep‐water layers, phytoplankton were growing at nutrient‐saturated rates, indicating that phytoplankton growth was more likely light limited. In the epilimnion, recycled and internal sources were important for both N and P in different experiments, but the importance of various supply sources did not systematically differ for N and P. In epilimnetic experiments, there was strong experiment‐to‐experiment variation in contributions of recycling sources of N and P, suggesting that resupply of N and P via grazers was decoupled. Comparison of phytoplankton responses to nutrient deletion in undiluted vs. highly diluted treatments indicated that inferences regarding frequency and magnitude of nutrient limitation, as well as identity of the primary limiting nutrient, depended on dilution.