Realism of model ecosystems: an evaluation of physicochemistry and macroinvertebrate assemblages in artificial streams

Ecological realism is an important yet rarely reported feature of model ecosystems. In this case study, we assess the realism of four outdoor artificial stream mesocosms (4 m²) bordering a chalk river in southern England. Comparisons of physiochemical conditions and benthic macroinvertebrate assemblages were made between the mesocosm units and the parent water body, a side arm of the River Frome. Physicochemistry of the mesocosm replicates was similar to that of the source stream, with congruent temporal variation evident between the real system and each of the models. The high realism of the mesocosms was explained by the outdoor location and close physical proximity of the array to the source stream, and the short mesocosm residence time of water sourced from the parent feeder system. Mesocosms supported a diverse array of benthic macroinvertebrates (60 families from 14 taxonomic orders), including all macroinvertebrate families in the source stream. Individual mesocosms contained a mean of 89% of source stream biota. We conclude that once-through mesocosms can be satisfactory analogues of natural systems, particularly where model and natural scales overlap. Handling editor: D. Dudgeon     

Responses of algae, bacteria, Daphnia and natural parasite fauna of Daphnia to nutrient enrichment in mesocosms

Understanding responses of parasites to changes in nutrient regimes is necessary for prediction of their role in aquatic ecosystems under global change in nutrient loading. We studied the response of the natural parasite fauna of Daphnia longispina to nutrient enrichment in mesocosms in a small humic lake. We measured the concentrations of inorganic phosphorus and nitrogen in the water, total nutrients in the seston, algal and bacterial biomass, Daphnia population dynamics, Daphnia stoichiometry, Daphnia stable isotope values and the presence and abundance of parasites in treated mesocosms as compared to three control ones. Incorporation of the nutrient enrichment in the food web was seen as increased nutrient concentrations in the epilimnion and as a decrease in carbon:nutrient ratios and δ¹⁵N values in Daphnia. Nutrient enrichment did not significantly influence algal, bacterial or Daphnia biomass. One of the four parasite species observed, unidentified small gut parasite, had a higher prevalence (percentage of Daphnia infected) in treated mesocosms, but its intensity (number of parasites per infected host) remained the same among treatments. Our results suggest that the effect of nutrient enrichment on host–parasite dynamics is dependent on complex interactions within food webs and on the epidemiological traits of parasites.     

Fatty Acid Composition at the Base of Aquatic Food Webs Is Influenced by Habitat Type and Watershed Land Use

Spatial variation in food resources strongly influences many aspects of aquatic consumer ecology. Although large-scale controls over spatial variation in many aspects of food resources are well known, others have received little study. Here we investigated variation in the fatty acid (FA) composition of seston and primary consumers within (i.e., among habitats) and among tributary systems of Lake Michigan, USA. FA composition of food is important because all metazoans require certain FAs for proper growth and development that cannot be produced de novo, including many polyunsaturated fatty acids (PUFAs). Here we sampled three habitat types (river, rivermouth and nearshore zone) in 11 tributaries of Lake Michigan to assess the amount of FA in seston and primary consumers of seston. We hypothesize that among-system and among-habitat variation in FAs at the base of food webs would be related to algal production, which in turn is influenced by three land cover characteristics: 1) combined agriculture and urban lands (an indication of anthropogenic nutrient inputs that fuel algal production), 2) the proportion of surface waters (an indication of water residence times that allow algal producers to accumulate) and 3) the extent of riparian forested buffers (an indication of stream shading that reduces algal production). Of these three land cover characteristics, only intense land use appeared to strongly related to seston and consumer FA and this effect was only strong in rivermouth and nearshore lake sites. River seston and consumer FA composition was highly variable, but that variation does not appear to be driven by the watershed land cover characteristics investigated here. Whether the spatial variation in FA content at the base of these food webs significantly influences the production of economically important species higher in the food web should be a focus of future research.     

Response of a Stoichiometrically Imbalanced Ecosystem to Manipulation of Nutrient Supplies and Ratios

Cuatro Ciénegas Basin (CCB) is a desert ecosystem that hosts a large diversity of water bodies. Many surface waters in this basin have imbalanced nitrogen (N) to phosphorus (P) stoichiometry (total N:P > 100 by atoms), where P is likely to be a limiting nutrient. To investigate the effects of nutrient stoichiometry on planktonic and sediment ecosystem components and processes, we conducted a replicated in situ mesocosm experiment in Lagunita, a shallow pond located in the southwest region of the basin. Inorganic N and P were periodically added to mesocosms under three different N:P regimes (P only, N:P = 16 and N:P = 75) while the control mesocosms were left unamended. After three weeks of fertilization, more than two thirds of the applied P was immobilized into seston or sediment. The rapid uptake of P significantly decreased biomass C:P and N:P ratios, supporting the hypothesis that Lagunita is P-limited. Meanwhile, simultaneous N and P enrichment significantly enhanced planktonic growth, increasing total planktonic biomass by more than 2-fold compared to the unenriched control. With up to 76% of added N sequestered into the seston, it is suspected that the Lagunita microbial community also experienced strong N-limitation. However, when N and P were applied at N:P = 75, the microbes remained in a P-limitation state as in the untreated control. Two weeks after the last fertilizer application, seston C:P and N:P ratios returned to initial levels but chlorophyll a and seston C concentrations remained elevated. Additionally, no P release from the sediment was observed in the fertilized mesocosms. Overall, this study provides evidence that Lagunita is highly sensitive to nutrient perturbation because the biota is primarily P-limited and experiences a secondary N-limitation despite its high TN:TP ratio. This study serves as a strong basis to justify the need for protection of CCB ecosystems and other low-nutrient microbe-dominated systems from anthropogenic inputs of both N and P.     

Effects of the benthic suspension feeder Dreissena polymorpha on zooplankton in a large river

1. We conducted a series of in situ enclosure experiments to assess the impact of zebra mussels ( Dreissena polymorpha ) on the plankton of the Ohio River. Adult mussels were suspended in pelagic enclosures ('potamocorrals') at three densities (0, 1000, 2500 mussels per corral) and incubated for 6 days with daily plankton and physiochemical sampling.
2. The presence of adult zebra mussels was correlated with a shift in composition of the phytoplankton community and a severe reduction in some rotifers. The effects of zebra mussels on the larger zooplankton were taxon-dependent, but bacterial density showed no trend among treatments.
3. Zebra mussels may have significant negative impacts on zooplankton, which may in turn alter riverine food webs.     

Comparing Grazing on Lake Seston by Dreissena and Daphnia: Lessons for Biomanipulation

Biomanipulation measures in lakes, taken to diminish algal blooms, have mainly been restricted to the reduction of zooplanktivorous fish with the aim to stimulate the grazing pressure by native filter feeders such as Daphnia. However, larger filter feeders like the exotic zebra mussel, Dreissena polymorpha, have been suggested as an optional tool because of their high filtering capacity. We compared grazing by two filter feeders, D. polymorpha and Daphnia galeata, offered seston from Lake IJsselmeer, the Netherlands in two consecutive years: 2002 and 2003. The seston in both years was dominated by the colony-forming cyanobacterium Microcystis aeruginosa. The grazing studies were performed under controlled conditions in the laboratory and samples were analyzed on a flow cytometer, making it possible to quantify grazing on different seston components and size fractions, including cyanobacteria, other phytoplankton (green algae, diatoms, etc.), and detritus. No differences in clearance rates, on a per weight basis, were found between the two grazer species. The clearance rate on cyanobacteria (especially <20 μm) was lower in 2003 than in 2002. In 2003, the microcystin concentration of cyanobacteria was higher than in 2002, suggesting that the observed lower clearance rate in 2003 was due to the enhanced toxin content of the cyanobacteria. Zebra mussels, although indiscriminately filtering all seston groups out of the water, positively selected for phytoplankton in their mantle cavity, irrespective of its toxicity, and rejected detritus. Since no differences in clearance rates were found between the two grazer species, we conclude that for biomanipulation purposes of shallow lakes, native species like the daphnids should be preferred over exotic species like zebra mussels. When the seston is dominated by phytoplankton that cannot be filtered out of the water column by Daphnia, however, the use of zebra mussels may be considered. Care should be taken, however, in the choice of the lakes since the mussels may have severe ecological and economic impacts.     

Physiological energetics of the zebra musselDreissena polymorpha in lakes III. Metabolism and net growth efficiency

Oxygen consumption and ammonia excretion of the zebra musselDreissena polymorpha from 3 sites in lakes were estimated regularly over the course of 1 1/2 years at ambient temperature. They showed a pronounced annual cycle, when expressed in absolute terms (at standard shell length) and in weight specific terms (at standard tissue weight). The atomic ratio of oxygen consumed to ammonia-N released (O/N ratio) was lowest in late summer at all sites (10 to 20) and highest during winter and spring (50 to > 100). The mean body weight exponent pooled from these sites was 0.78 (95% confidence interval±0.07) for the oxygen consumption rate and 0.80 (confidence interval±0.10) for the ammonia excretion rate. Both oxygen consumption and ammonia excretion were significantly correlated with the water temperature at the 2 shallow water sites, where temperature variation was most pronounced. Correlation with seston content or gonad volume were insignificant at these sites. The quotient of filtration capacity to oxygen consumption rate was about 3 times higher at the site with the poorest food conditions compared to the other sites. Net growth efficiency was highly variable; its annual average was 35 to 40 per cent and independent of locality and animal size.     

Physiological energetics of the zebra musselDreissena polymorpha in lakes II. Food uptake and gross growth efficiency

Food uptake ofDreissena polymorpha from two shallow water sites and one site at a 9 m depth in lakes was monitored over the course of 1 1/2 years. Feeding was characterized by the filtration capacity and ingestion capacity (i.e. the unrestricted maximum rates) at ambient temperature. The available food was estimated as seston dry weight, as seston ash free dry weight and as seston volume (sum of the volumes of the particles in the water column). The filtration capacity and ingestion capacity varied strongly during the course of the year by more than 1:10, with maximum values during late spring and early summer. The filtration capacity was correlated with water temperature, the ingestion capacity with temperature and seston concentration. However, at identical water temperatures both capacities were higher in spring than in autumn. At the 9 m site, filtration and ingestion capacity was also significantly influenced by lack of oxygen caused by stratification during summer. In the allometric equations by which both capacities (C) were related to body weightW (C=a W ^b ), an exponent b of 0.89±0.14 (mean±95% confidence interval) was calculated for the filtration capacity and 1.00±0.18 for the ingestion capacity. Both exponents did not vary significantly during the course of the year. The seasonal trend of the incipient limiting concentration (i.e. the seston concentration at which the intestine is filled to its capacity when the animal filters at maximum rate) was less pronounced. It showed a significantly positive correlation with seston concentration and a negative correlation with temperature. Except during the spring bloom, the incipient limiting concentration was in the range of the seston volume registered at the sites. Gross growth efficiencies were highly variable. An annual average between 5 and 20% was calculated. Growth efficiencies were lower at the shallow water sites with the highest seston concentrations.     

Effects of trophic skewing of species richness on ecosystem functioning in a diverse marine community

Widespread overharvesting of top consumers of the world's ecosystems has "skewed" food webs, in terms of biomass and species richness, towards a generally greater domination at lower trophic levels. This skewing is exacerbated in locations where exotic species are predominantly low-trophic level consumers such as benthic macrophytes, detritivores, and filter feeders. However, in some systems where numerous exotic predators have been added, sometimes purposefully as in many freshwater systems, food webs are skewed in the opposite direction toward consumer dominance. Little is known about how such modifications to food web topology, e.g., changes in the ratio of predator to prey species richness, affect ecosystem functioning. We experimentally measured the effects of trophic skew on production in an estuarine food web by manipulating ratios of species richness across three trophic levels in experimental mesocosms. After 24 days, increasing macroalgal richness promoted both plant biomass and grazer abundance, although the positive effect on plant biomass disappeared in the presence of grazers. The strongest trophic cascade on the experimentally stocked macroalgae emerged in communities with a greater ratio of prey to predator richness (bottom-rich food webs), while stronger cascades on the accumulation of naturally colonizing algae (primarily microalgae with some early successional macroalgae that recruited and grew in the mesocosms) generally emerged in communities with greater predator to prey richness (the more top-rich food webs). These results suggest that trophic skewing of species richness and overall changes in food web topology can influence marine community structure and food web dynamics in complex ways, emphasizing the need for multitrophic approaches to understand the consequences of marine extinctions and invasions.     

Food web complexity and allometric scaling relationships in stream mesocosms: implications for experimentation

1. Mesocosms are used extensively by ecologists to gain a mechanistic understanding of ecosystems based on the often untested assumption that these systems can replicate the key attributes of natural assemblages. 2. Previous investigations of stream mesocosm utility have explored community composition, but here for the first time, we extend the approach to consider the replicability and realism of food webs in four outdoor channels (4 m(2)). 3. The four food webs were similarly complex, consisting of diverse assemblages (61-71 taxa) with dense feeding interactions (directed connectance 0.09-0.11). Mesocosm food web structural attributes were within the range reported for 82 well-characterized food webs from natural streams and rivers. When compared with 112 additional food webs from standing freshwater, marine, estuarine and terrestrial environments, stream food webs (including mesocosms) had similar characteristic path lengths, but typically lower mean food chain length and exponents for the species-link relationship. 4. Body size (M) abundance (N) allometric scaling coefficients for trivariate taxonomic mesocosm food webs (-0.53 to -0.49) and individual size distributions (-0.60 to -0.58) were consistent and similar to those from natural systems, suggesting that patterns of energy flux between mesocosm consumers and resources were realistic approximations. 5. These results suggest that stream mesocosms of this scale can support replicate food webs with a degree of biocomplexity that is comparable to 'natural' streams. The findings highlight the potential value of mesocosms as model systems for performing experimental manipulations to test ecological theories, at spatiotemporal scales of relevance to natural ecosystems.     

Primary consumers as bioindicator of nitrogen pollution in lake planktonic and benthic food webs

The increased nitrogen loading from anthropogenic sources has affected aquatic ecosystems and has cascaded through food webs worldwide. Therefore, the evaluation of ecological impacts of anthropogenic nitrogen has become increasingly important. In this paper, we investigated the effect of nutrient enrichment in the planktonic and benthic food webs of a hypereutrophic urban lake using stable nitrogen isotope analysis. The stable isotope mixing model revealed that zooplanktons rely mainly on planktonic nitrogen, and that Chironomus and Oligochaete rely mainly on benthic nitrogen. The stable nitrogen isotope signatures of seston and the sediment organic matter can be a sensitive indicator, because they are correlated with the nitrogen content of water and sediment. Furthermore, the relationships between the stable nitrogen isotope values of primary production and primary consumers indicate that polluted nitrogen has transferred through the planktonic and benthic food webs, respectively. For primary consumers, the stable nitrogen isotope values of zooplanktons were correlated to nitrogen concentrations of water, and the stable nitrogen values of Chironomus and Oligochaete were correlated to the nitrogen content of sediment, respectively. This finding indicates that these primary consumers can be used as promising bioindicators of the anthropogenic nitrogen input in planktonic and benthic food webs, respectively. Our results suggest that stable nitrogen isotope of primary consumers can act as bioindicators to detect the dispersal patterns of anthropogenic contamination and understand the incorporation into and movement of waste nitrogen in pelagic and benthic food chains.     

Contrasting wetland CH4 emission responses to simulated glacial atmospheric CO2 in temperate bogs and fens

Wetlands were the largest source of atmospheric methane (CH(4) ) during the Last Glacial Maximum (LGM), but the sensitivity of this source to exceptionally low atmospheric CO(2) concentration ([CO(2) ]) at the time has not been examined experimentally. We tested the hypothesis that LGM atmospheric [CO(2) ] reduced CH(4) emissions as a consequence of decreased photosynthate allocation to the rhizosphere. We exposed minerotrophic fen and ombrotrophic bog peatland mesocosms to simulated LGM (c. 200 ppm) or ambient (c. 400 ppm) [CO(2) ] over 21 months (n = 8 per treatment) and measured gaseous CH(4) flux, pore water dissolved CH(4) and volatile fatty acid (VFA; an indicator of plant carbon supply to the rhizosphere) concentrations. Cumulative CH(4) flux from fen mesocosms was suppressed by 29% (P < 0.05) and rhizosphere pore water [CH(4) ] by c. 50% (P < 0.01) in the LGM [CO(2) ], variables that remained unaffected in bog mesocosms. VFA analysis indicated that changes in plant root exudates were not the driving mechanism behind these results. Our data suggest that the LGM [CO(2) ] suppression of wetland CH(4) emissions is contingent on trophic status. The heterogeneous response may be attributable to differences in species assemblage that influence the dominant CH(4) production pathway, rhizosphere supplemented photosynthesis and CH(4) oxidation.     

Replicability of physicochemistry and macroinvertebrate assemblages in stream mesocosms: implications for experimental research

1. The replicability of mesocosms is assessed in a case study of artificial streams (4 m²) flanking a lowland chalk river in southern U.K. Among‐mesocosm comparisons of physicochemistry and macroinvertebrate assemblage composition (in drift and benthos) were made during a 2‐year survey of 12 outdoor once‐through linear channels. 2. Mesocosm physicochemistry was highly replicable, with statistically significant congruence in temporal variation across the mesocosm series, low spatial variation and no significant blocking or positional effects arising from mesocosm layout. Parallel physicochemistry was attributed to the outdoor stream‐side location and short water residence times. 3. Mesocosms were colonized by 127 macroinvertebrate taxa from 15 taxonomic orders. Both benthic and drifting assemblages were comparable among mesocosms, with no significant effect of mesocosm layout. Small differences in taxonomic composition were, however, evident among particular mesocosms, with higher (e.g. Tinodes, Limnius and Elmis) or lower (e.g. Pisidium and Valvata) abundances observed for a minority (5%) of taxa. We conclude that large (4 m²) outdoor flow – through mesocosms can be replicable when located near to the source system and allowed to establish naturally.     

A stable carbon and nitrogen isotope study of lake food webs in Canada's Boreal Plain

1.  Stable carbon and nitrogen isotope and fish stomach content analyses were used to investigate food webs in five relatively undisturbed lakes on the Boreal Plain of Canada. Stable isotope analysis was also used to determine the importance of external and internal carbon sources.
2.  Overlap in the carbon and nitrogen signatures of primary producers made it difficult to determine unambiguously the feeding habits of many invertebrates. However, isotope analysis suggested that external carbon inputs were detectable in the aquatic food chains of the one lake with a short water residence time («1 year). In the other four lakes, with water residence times ≥1 year, autochthonous carbon was the only detectable carbon source in the food webs.
3.  Food webs in these lakes spanned a range of four to five trophic levels. Both invertebrates and fish appeared to eat a variety of food, often feeding at more than one trophic level.
4.  With the exception of one lake (SPH20), top predators in these lakes, northern pike ( Esox lucius ) and fathead minnows ( Pimephales promelas ), occupied similar trophic positions despite large differences in body size and trophic morphology. In SPH20, where there were two additional fish species, pike occupied a higher trophic position. However, all the top predators in each lake appeared to be omnivores and generalists.
5.  The prevalence of omnivory and the apparent generalist feeding habits of fish in these lakes suggest that organisms are flexible in their feeding habits and that these food webs will be resilient to disturbance.     

Linking major nutrients (C,H, N,and P) to trace metals (Fe,Mn, and Cu) in lake seston in southern Brazil

We investigated the relationships between major nutrients (C, H, N, and P) and trace metals (Cu, Fe, and Mn) in seston samples from ten lake/lagoon systems in southern Brazil. The systems were characterized by a diverse set of limnological features, including surface areas from 10⁻¹ to 10² km², water color, a _CDOM(440), from 1.4 to 12.9 m⁻¹, and electrical conductivity from 50 to 100 000 μS cm⁻¹. Seston concentrations also varied a great deal, 32-fold. The elemental (C: N, C: P, and N: P) and C: Chl-a ratios in the seston samples indicated, however, common features; i.e., most of the lakes were N-and/or P-limited, and the seston organic fraction was composed of nonvascular plants (e.g., phytoplankton). Our intersystem comparison revealed that the relative content of organic matter in seston and seston concentrations in lake water tended to correlate positively and negatively, respectively, with trace metal concentrations across the seston samples. Possible influences of elemental and C: Chl-a ratios on the association of metals with seston matrices, although theoretically important, were only partially evidenced here; positive correlations were found between C: N and also Org-H: N ratios with trace metal concentrations. We speculate that such results could be circumstantial, as the nature of the seston matrices appeared to be very similar among them. This hypothesis should thus be the theme of further research. In short, our findings suggest that C: N and Org-H: N ratios as well as the relative content of major nutrients in seston and seston concentrations can be importantly related to trace metal concentrations in seston samples. In discussing the results, however, we consider that metal-seston relationships depend on a variety of physical, chemical, and biological factors and/or variables other than those measured in this study, which could also contribute for defining and explaining variations in metal-seston concentrations in lake ecosystems.     

Bottom‐up effects of lake sediment on pelagic food‐web compartments: a mesocosm study

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Terrestrialization alters organic matter dynamics and habitat quality for freshwater mussels (Unionoida) in floodplain backwaters

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Seasonal changes in the biochemistry of lake seston

1. The quantity of seston was measured and the elemental carbon, nitrogen and phosphorus (C, N, P) and biochemical composition (carbohydrate, protein, lipid) of the < 53 μm size fraction in three temperate lakes during one year was analysed. The lakes differed in nutrient concentration and were characterized as oligotrophic, mesotrophic and eutrophic. Linear regression analyses defined associations between seston composition and either lake trophic status, depth or season. 2. The concentration of particulate organic seston was greatest during spring and autumn and lowest during the clear water period in early summer. Seasonal patterns in seston elemental and biochemical percentage composition (quality) were observed to be independent of differences in seston quantity. 3. Concentrations of seston C, N and P were high in most cases in the spring and autumn and low in summer. Concentrations of P were particularly high during late summer and early autumn in the metalimnion, perhaps because of recovery of P from anaerobic sediments and hypolimnetic waters. Because seston C and N did not increase as markedly as P, C : P and N : P ratios both declined in the autumn. Primary production was thought to be co-limited by N and P in all three of these lakes; however, the data suggested that N might be more important as a major limiting nutrient in the eutrophic lake as the metalimnion increased in depth in late summer and autumn. 4. Concentrations of protein, carbohydrate, polar lipid and triglyceride generally increased with lake type as expected (greatest in the eutrophic lake), but showed no relationship with water depth. As the year progressed, no significant changes were measured in protein and carbohydrate concentrations; however, the concentration of polar lipid decreased and triglyceride increased significantly with time of year. 5. The biochemical composition of seston varied during the year and among lakes; for example, in Lake Waynewood the proportion of protein composing the seston (percentage protein by weight) varied from < 10% to > 40%. No statistically significant patterns in the percentage protein or carbohydrate were found. However, the proportion of seston comprised of triglyceride decreased with lake type and increased during the year; whereas the proportion of seston as polar lipid increased with lake type and decreased during the year. Triglyceride comprised most of the lipid. Both protein : lipid and protein : carbohydrate ratios tended to be greatest in summer and lowest in the spring and autumn. 6. Relationships between samples and biochemical composition analysed by Canonical Correspondence Analysis (Canoco) indicated similar patterns in seasonal changes in seston biochemistry for the three lakes, with samples separated primarily by vectors for lake type (oligotrophic to eutrophic) and the percentage polar lipid (proportion of total lipid) and secondarily by vectors for date and water depth (epilimnion or metalimnion). 7. These seasonal biochemical changes in the seston food base were compared with biochemical changes known to occur in algae grown under N-or P-limited conditions in the laboratory, and the resultant quality of this algal food for suspension-feeding consumers (zooplankton). It was concluded that zooplankton were likely to be physiologically challenged by these distinct seasonal shifts in the quality of lake seston.     

Determinants of seston C : P-ratio in lakes

1. The ratio of carbon to phosphorus (C : P) in seston is a major determinant of energy transfer in aquatic food webs and may vary more than an order of magnitude owing to various extrinsic and intrinsic factors. In this study, the determinants of C : P‐ratios in lake particulate matter (seston) was assessed in 112 Norwegian lakes, covering a C : P (atomic ratio) from 24 to 1842 (mean 250). 2. No overall effects of lake area, season or latitude on C : P was detected. Particulate P, but not particulate C, correlated with C : P. Multivariate analysis including a range of lake properties revealed total dissolved P, as the major determinant of sestonic C : P, with the fraction of detritus in total seston, chlorophyll or Secchi depth and lake colour as significant contributors. Together these parameters explained 30% of observed variance if using dissolved P and 81% if using total P as input variable to the multivariate model. 3. Chlorophyll and Secchi depth were highly correlated and substitutable in the analysis. Phytoplankton community composition did not affect seston C : P, probably reflecting the fact that live phytoplankton generally contributed <25% of the seston pool. 4. Total P correlated positively with C : P and is the key determinant of phytoplankton biomass and thus Secchi depth; the latter parameters contributed negatively to seston C : P, probably owing to increased light attenuation. These lake data thus support the light : nutrient ratio hypothesis, i.e. that high light and low P cause skewed uptake ratios of C to P. 5. Zooplankton biomass in general and Daphnia biomass in particular, was negatively correlated with C : P, probably reflecting a negative impact of poor seston quality at high C : P. Zooplankton grazing and nutrient recycling may also have contributed to a negative correlation between zooplankton biomass and sestonic C : P.