Whole-lake dissolved inorganic 13C additions reveal seasonal shifts in zooplankton diet

Sustained whole-lake additions of 13C-enriched dissolved inorganic carbon (DIC), intended to increase experimentally the delta13C of DIC in the epilimnion of a small lake with high dissolved organic carbon (DOC), were made during three seasonal periods (spring, summer, and autumn). Coupled with carbon and nitrogen stable isotope analysis of zooplankton and several of their putative food sources, these additions were used to investigate seasonal changes in the relative contributions of different food sources to zooplankton diet in the lake. Four main potential food sources were considered: phytoplankton, heterotrophic bacteria (HB), methanotrophic bacteria (MOB), and green sulfur bacteria (GSB). Because the number of potential food sources exceeded the number of isotopes analyzed, a computer program (IsoSource) was used to estimate the range of possible contributions of the various food sources. During all three periods the added inorganic 13C quickly increased the epilimnetic DIC delta13C by between 18 per thousand and 21 per thousand above the initial value of approximately -21 per thousand. This 13C enrichment of DIC was rapidly transmitted to the particulate organic matter (POM), which included photosynthetic phytoplankton. In spring and summer, delta13C of both adult and juvenile Daphnia increased by approximately 10 per thousand, indicating that Daphnia utilized autochthonous carbon. However, this 13C labeling of Daphnia was not so obvious during the autumn period, when their delta13C generally decreased. According to the IsoSource model outputs based on both delta13C and delta15N values, Daphnia utilized all four potential food source types during spring, summer, and autumn, but in different proportions. The possible contribution of phytoplankton to Daphnia diet was substantial (25-71%) in all seasons. The possible contributions of the bacterial food sources were more variable. The possible contribution of GSB was minor (0-20%) at all times and negligible in autumn. The possible contribution of HB was higher but very variable. Methanotrophic bacteria always made a significant contribution to Daphnia diet and were likely the single most important food source in autumn. Since both HB and MOB in this high-DOC lake probably depend largely on allochthonous organic carbon, our results highlight the seasonal variability in the potential importance of ecosystem subsidies in lake food webs.     

Terrestrial, benthic, and pelagic resource use in lakes: results from a three-isotope Bayesian mixing model

Fluxes of organic matter across habitat boundaries are common in food webs. These fluxes may strongly influence community dynamics, depending on the extent to which they are used by consumers. Yet understanding of basal resource use by consumers is limited, because describing trophic pathways in complex food webs is difficult. We quantified resource use for zooplankton, zoobenthos, and fishes in four low-productivity lakes, using a Bayesian mixing model and measurements of hydrogen, carbon, and nitrogen stable isotope ratios. Multiple sources of uncertainty were explicitly incorporated into the model. As a result, posterior estimates of resource use were often broad distributions; nevertheless, clear patterns were evident. Zooplankton relied on terrestrial and pelagic primary production, while zoobenthos and fishes relied on terrestrial and benthic primary production. Across all consumer groups terrestrial reliance tended to be higher, and benthic reliance lower, in lakes where light penetration was low due to inputs of terrestrial dissolved organic carbon. These results support and refine an emerging consensus that terrestrial and benthic support of lake food webs can be substantial, and they imply that changes in the relative availability of basal resources drive the strength of cross-habitat trophic connections.     

Quantity and quality: unifying food web and ecosystem perspectives on the role of resource subsidies in freshwaters

Although the study of resource subsidies has emerged as a key topic in both ecosystem and food web ecology, the dialogue over their role has been limited by separate approaches that emphasize either subsidy quantity or quality. Considering quantity and quality together may provide a simple, but previously unexplored, framework for identifying the mechanisms that govern the importance of subsidies for recipient food webs and ecosystems. Using a literature review of > 90 studies of open-water metabolism in lakes and streams, we show that high-flux, low-quality subsidies can drive freshwater ecosystem dynamics. Because most of these ecosystems are net heterotrophic, allochthonous inputs must subsidize respiration. Second, using a literature review of subsidy quality and use, we demonstrate that animals select for high-quality food resources in proportions greater than would be predicted based on food quantity, and regardless of allochthonous or autochthonous origin. This finding suggests that low-flux, high-quality subsidies may be selected for by animals, and in turn may disproportionately affect food web and ecosystem processes (e.g., animal production, trophic energy or organic matter flow, trophic cascades). We then synthesize and review approaches that evaluate the role of subsidies and explicitly merge ecosystem and food web perspectives by placing food web measurements in the context of ecosystem budgets, by comparing trophic and ecosystem production and fluxes, and by constructing flow food webs. These tools can and should be used to address future questions about subsidies, such as the relative importance of subsidies to different trophic levels and how subsidies may maintain or disrupt ecosystem stability and food web interactions.     

Terrestrial organic matter as subsidies that aid in the recovery of macroinvertebrates in industrially damaged lakes

The importance of allochthonous carbon to the productivity of stream ecosystems in temperate ecozones is well understood, but this relationship is less established in oligotrophic lakes. The nearshore littoral zones, at the interface of terrestrial and aquatic systems, are areas where the influence of terrestrial subsidies is likely greatest. We investigated the response of nearshore communities to variation in the quantity and composition of allochthonous materials, determined the landscape characteristics that regulate the variation of this subsidy, and explored the potential for terrestrial restoration practices to influence the export of organic matter to lakes. Stepwise multiple regressions revealed that diversity of nearshore macroinvertebrate families increased with the amount of fine particulate organic matter (FPOM) captured in sediment traps. The quantity of FPOM (g) increased with forest cover, and the relative amount of FPOM (percentage of total particulate material) in the traps increased with surface area of wetland in the catchments. These models suggest that terrestrially derived subsidies are important in smelter-impacted watersheds, and that the restoration of forests and wetlands will speed the return of nearshore consumer community diversity in industrially damaged lakes.     

Zooplankton biodiversity and lake trophic state: explanations invoking resource abundance and distribution

While empirical studies linking biodiversity to local environmental gradients have emphasized the importance of lake trophic status (related to primary productivity), theoretical studies have implicated resource spatial heterogeneity and resource relative ratios as mechanisms behind these biodiversity patterns. To test the feasibility of these mechanisms in natural aquatic systems, the biodiversity of crustacean zooplankton communities along gradients of total phosphorus (TP) as well as the vertical heterogeneity and relative abundance of their phytoplankton resources were assessed in 18 lakes in Quebec, Canada. Zooplankton community richness was regressed against TP, the spatial distribution of phytoplankton spectral groups, and the relative biomass of spectral groups. Since species richness does not adequately capture ecological function and life history of different taxa, features which are important for mechanistic theories, relationships between zooplankton functional diversity (FD) and resource conditions were examined. Zooplankton species richness showed the previously established tendency to a unimodal relationship with TP, but functional diversity declined linearly over the same gradient. Changes in zooplankton functional diversity could be attributed to changes in both the spatial distribution and type of phytoplankton resource. In the studied lakes, spatial heterogeneity of phytoplankton groups declined with TP, even while biomass of all groups increased. Zooplankton functional diversity was positively related to increased heterogeneity in cyanobacteria spatial distribution. However, a smaller amount of variation in functional diversity was also positively related to the ratio of biomass in diatoms/chrysophytes to cyanobacteria. In all observed relationships, a greater variation of functional diversity than species richness measures was explained by measured factors, suggesting that functional measures of zooplankton communities will benefit ecological research attempting to identify mechanisms behind environmental gradients affecting diversity.     

Reciprocal subsidies between freshwater and terrestrial ecosystems structure consumer resource dynamics

Cross-ecosystem movements of material and energy, particularly reciprocal resource fluxes across the freshwater-land interface, have received major attention. Freshwater ecosystems may receive higher amounts of subsidies (i.e., resources produced outside the focal ecosystem) than terrestrial ecosystems, potentially leading to increased secondary production in freshwaters. Here we used a meta-analytic approach to quantify the magnitude and direction of subsidy inputs across the freshwater-land interface and to determine subsequent responses in recipient animals. Terrestrial and freshwater ecosystems differed in the magnitude of subsidies they received, with aquatic ecosystems generally receiving higher subsidies than terrestrial ecosystems. Surprisingly, and despite the large discrepancy in magnitude, the contribution of these subsidies to animal carbon inferred from stable isotope composition did not differ between freshwater and terrestrial ecosystems, likely due to the differences in subsidy quality. The contribution of allochthonous subsidies was highest to primary consumers and predators, suggesting that bottom-up and top-down effects may be affected considerably by the input of allochthonous resources. Future work on subsidies will profit from a food web dynamic approach including indirect trophic interactions and propagating effects.     

A model of macroinvertebrate trophic structure and oxygen demand in freshwater wetlands

We present a model of macroinvertebrate trophic structure, detrital cycling, and dissolved oxygen (DO) dynamics in shallow freshwater wetlands with varying allochthonous subsidies. The model is based on field data of primary and secondary production in municipal wastewater-fed and river-fed constructed wetlands in central Ohio, USA. State variables for primary production include macrophyte, periphyton, and metaphyton biomass. Macroinvertebrate biomass is segregated by functional feeding group and includes collectors, scrapers, shredders, and predators. Model simulations demonstrate the association of water column dissolved oxygen, primary production, allochthonous organic matter, and the structure of the macroinvertebrate community. The quality and quantity of allochthonous carbon is shown to have considerable importance, not only as a food source but also as an oxygen sink. Allochthonous carbon equivalent to 5% of autochthonous production increases the macroinvertebrate standing crop by 4–17%, depending on particle size. A large allochthonous subsidy also reduces the simulated average diel dissolved oxygen and increases the percentage of hypoxia-tolerant macroinvertebrates. Simulations show both the heterotrophic response and the changes in community structure brought about by an allochthonous subsidy.     

Effects of buoyancy, transparency and zooplankton feeding on surface maxima and deep maxima: Comprehensive mathematical model for vertical distribution in cyanobacterial biomass

Inhomogeneous vertical distributions of the cyanobacterial biomass are widely observed during the summer season in stratified lake ecosystems. Among these are surface maxima characterized by surface scum formation and deep or subsurface maxima also known as deep chlorophyll maxima (DCM). The former occurs at the epilimnion in eutrophic lakes, and are usually caused by colonial cyanobacteria such as Microcystis. On the other hand, the latter occurs at the metalimnion and the upper part of the hypolimnion near the thermocline in oligotrophic lakes, and are referred to filamentous cyanobacteria such as Oscillatoria. The aim of this paper is to present a simple mathematical model that can simultaneously describe these phenomena including the annual and diurnal variations, emphasizing the roles of buoyancy regulation, transparency of the lake and zooplankton feeding on cyanobacteria. According to our computer analyses, the increased buoyancy, the low clarity of the lake and the low rate of zooplankton feeding take significant roles in formation of surface maxima, while the reversal of these factors makes deep maxima predominant. Our two-component model with nutrients and cyanobacteria can distinguish between two phenomena by changing the parameters for these factors, without altering the model itself.     

Metabolic rates of epipelagic marine zooplankton as a function of body mass and temperature

The metabolic rates (oxygen uptake, ammonia excretion, phosphate excretion) of epipelagic marine zooplankton have been expressed as a function of body mass (dry, carbon, nitrogen and phosphorus weights) and habitat temperature, using the multiple-regression method. Zooplankton data used for this analysis are from phylogenetically mixed groups (56 to 143 species, representing 7 to 8 phyla, body mass range: 6 orders of magnitude) from various latitudes (habitat temperature range:-1.4° to 30°C). The results revealed that 84 to 96% of variation in metabolic rates is due to body mass and habitat temperature. Among the various body-mass units, the best correlation was provided by carbon and nitrogen units for all three metabolic rates. Oxygen uptake, ammonia excretion and phosphate excretion are all similar in terms of body-mass effect, but differ in terms of temperature effect. With carbon or nitrogen body-mass units, calculated Q₁₀ values are 1.82 to 1.89 for oxygen uptake, 1.91 to 1.93 for ammonia excretion and 1.55 for phosphate excretion. The effects of body mass and habitat temperature on the metabolic quotients (O:N, N:P, O:P) are insignificant. The present results for oxygen-uptake rate vs body mass do not differ significantly from those reported for general poikilotherms by Hemmingsen and for crustaceans by Ivleva at a comparable temperature (20°C). The importance of a body-mass measure for meaningful comparison is suggested by the evaluation of the habitat-temperature effect between mixed taxonomic groups and selected ones. Considering the dominant effects of body mass and temperature on zooplankton metabolic rates, the latitudinal gradient of community metabolic rate for net zooplankton in the ocean is estimated, emphasizing the non-parallelism between community metabolic rates and the standing stock of net zooplankton.     

Variability of particulate organic carbon and nitrogen in the Namibian upwelling system

Particulate organic carbon (POC) and nitrogen (PON) were analyzed during cruises undertaken in September 1985 and April 1986 along the Namibian upwelling system. The main objectives were to provide estimates of standing stocks of particulate organic matter (POM) and analyze its temporal and spatial variability. Average estimates of total carbon standing stock (0 to 100 m depth) accounted for 1.2 g-at C m^–2 during active and 1.32 g-at C m^–2 during abated upwelling. Zooplankton carbon was estimated as 0.22 and 0.27 g-at C m^–2, respectively, for both periods. POM was generally concentrated near the surface, especially during abated upwelling. During abated upwelling, POM was not only abundant inshore but also over the shelf, as a response of a more diffuse frontal system and a more strongly stratified water column. Cross-shelf gradients were more significant during active upwelling, while alongshore gradients accounted for most of the variance of particulate organic matter during abated periods. This result was a consequence of the seasonal intrusion of warm, Angolan water from the north during the period of minimum upwelling, and resulted in poorer POM concentrations and higher consumer: producer ratios (24.4%). Nevertheless, this last conclusion should be regarded with caution due to the lack of comparative interannual variability. A 48 h study at a fixed station permitted analysis of the daily variability in POM during the intrusion process. Changes in the thickness of the surface mixed layer due to irregular time-spaced pulses of non-homogeneous water masses resulted in sudden enrichments and renewals of phytoplankton and zooplankton populations in a matter of hours.     

Independent gradients of producer, consumer, and microbial diversity in lake plankton

Interactions between trophic levels during food web assembly can drive positive correlations in diversity between producers, consumers, and decomposers. However, the contribution of trophic interactions relative to local environmental factors in promoting species diversity is poorly understood, with many studies only considering two trophic levels. Here we examine correlations in diversity among zooplankton, phytoplankton, and bacteria in the pelagic zone of 31 lakes in British Columbia, Canada. We sampled species diversity of zooplankton and phytoplankton through morphological identification, and bacterial genetic diversity was estimated by denaturing gradient gel electrophoresis (DGGE) of 16S rDNA polymorphisms. We looked for correlations in diversity that were independent of the abiotic environment by statistically controlling for 18 limnological variables. No significant correlations were found between the diversity of zooplankton, phytoplankton, and bacteria. In addition, the physical factors that were associated with species composition in one trophic level were independent of those that were important for another. Our results provide no support for the importance of direct feedbacks between producers, consumers, and decomposers in maintaining diversity. Zooplankton, phytoplankton, and bacterial diversity and composition are regulated independently from one another and respond to different environmental variables. These results suggest that species of lake plankton show loose trophic associations with one another due to broad diets in consumers and decomposers.     

Is catchment productivity a useful predictor of taxa richness in lake plankton communities?

The influence of catchment variables on lake organisms is understudied. The terrestrial zone in the vicinity of lakes is, however, probably highly important for biota due to the effects on water chemistry and to various processes operating across ecosystem boundaries. We examined the relative importance of lake and catchment variables, as well as large-scale geographical factors, on the taxa richness of phyto- and zooplankton in 100 small lakes in Finland. In variation partitioning, the variability of phytoplankton richness was most strongly related to the effects of lake variables, the joint effects of lake and catchment variables, and the joint effects of all three groups of variables. Zooplankton richness, in turn, was most strongly related to the effects of lake and catchment variables and the joint effect of lake and catchment variables. The exact results of the variation partitioning depended on the catchment sizes considered in the regression models. Among lake variables, planktonic richness was strongly related to variables indicating productivity. Among catchment variables, the normalized difference vegetation index (NDVI), indicating catchment productivity, showed a relatively strong association with planktonic richness. These results provide evidence that catchment variables such as the NDVI may be efficient predictors of planktonic richness in small lakes. It is possible that individual lakes embedded in a highly productive landscape have higher taxa richness than solitary, potentially productive lakes because of the high influx of dispersing propagules from the regional pool. We also suggest that catchment variables may respond to environmental changes at different scales than the lake variables, and explicit consideration of catchment productivity would therefore be useful when planning research and monitoring programs for freshwater organisms.     

Evidence supporting the importance of terrestrial carbon in a large-river food web

Algal carbon has been increasingly recognized as the primary carbon source supporting large-river food webs; however, many of the studies that support this contention have focused on lotic main channels during low-flow periods. The flow variability and habitat-heterogeneity characteristic of these systems has the potential to significantly influence food web structure and must be integrated into models of large-river webs. We used stable-isotope analysis and IsoSource software to model terrestrial and algal sources of organic carbon supporting consumer taxa in the main channel and oxbow lakes of the Brazos River, Texas, USA, during a period of frequent hydrologic connectivity between these habitat types. Standardized sampling was conducted monthly to collect production sources and consumer species used in isotopic analysis. Predictability of hydrologic connections between habitat types was based on the previous 30 years of flow data. IsoSource mixing models identified terrestrial C3 macrophytes (riparian origin) as the primary carbon source supporting virtually all consumers in the main channel and most consumers in oxbow lakes. Small-bodied consumers (<100 mm) in oxbow lakes assimilated large fractions of algal carbon whereas this pattern was not apparent in the main channel. Estimates of detritivore trophic positions based on delta15N values indicated that terrestrial material was likely assimilated via invertebrates rather than directly from detritus. High flows in the river channel influenced algal standing stock, and differences in the importance of terrestrial and algal production sources among consumers in channel vs. oxbow habitats were associated with patterns of flooding. The importance of terrestrial material contradicts the findings of recent studies of large-river food webs that have emphasized the importance of algal carbon and indicates that there can be significant spatial, temporal, and taxonomic variation in carbon sources supporting consumers in large rivers.     

Monthly changes in the abundance and biomass of zooplankton and water quality parameters in Kukkarahalli Lake of Mysore, India

Zooplankton abundance and distribution are of ecological importance, as they are very sensitive to change, therefore zooplankton make ideal indicators of aquatic ecosystem. This study carried out on the abundance of rotifer, cladoceran, cyclopoid-copepod and ostracod zooplankton groups and biomass of total zooplankton were studied every month for one year. It is interesting and noteworthy to note that Calanoid and Harpacticoid zooplankton groups and free carbon dioxide were completely absent in all the four sampling sites throughout the study year. About 53% of the variation in the abundance of Cladoceran, 55% of variation in the Cyclopoid-copepod, 39% of variation in the ostracod and 53% of variation in the abundance of total zooplankton were mainly due to pH. Interestingly, negative relationship was found between the total zooplankton and concentration of phosphate as in this lake 67% decrease in wet biomass was mainly because of phosphate, where as 47% of dry biomass of total zooplankton was positively correlated with conductivity.     

Quantitative approaches to the analysis of stable isotope food web data

Ecologists use stable isotopes (delta13C, delta15N) to better understand food webs and explore trophic interactions in ecosystems. Traditionally, delta13C vs. delta15N bi-plots have been used to describe food web structure for a single time period or ecosystem. Comparisons of food webs across time and space are increasing, but development of statistical approaches for testing hypotheses regarding food web change has lagged behind. Here we present statistical methodologies for quantitatively comparing stable isotope food web data. We demonstrate the utility of circular statistics and hypothesis tests for quantifying directional food web differences using two case studies: an arthropod salt marsh community across a habitat gradient and a freshwater fish community from Lake Tahoe, USA, over a 120-year time period. We calculated magnitude and mean angle of change (theta) for each species in food web space using mean delta13C and delta15N of each species as the x, y coordinates. In the coastal salt marsh, arthropod consumers exhibited a significant shift toward dependence on Spartina, progressing from a habitat invaded by Phragmites to a restored Spartina habitat. In Lake Tahoe, we found that all species from the freshwater fish community shifted in the same direction in food web space toward more pelagic-based production with the introduction of nonnative Mysis relicta and onset of cultural eutrophication. Using circular statistics to quantitatively analyze stable isotope food web data, we were able to gain significant insight into patterns and changes in food web structure that were not evident from qualitative comparisons. As more ecologists incorporate a food web perspective into ecosystem analysis, these statistical tools can provide a basis for quantifying directional food web differences from standard isotope data.     

Stable C and N isotope analysis elucidated the importance of zooplankton in a tropical seagrass bed of Santiago Island,Northwestern Philippines

The role of zooplankton in a tropical seagrass ecosystem was investigated in milkfish farms pollution-impacted and -unimpacted seagrass beds in Santiago Island coral reefs, Northwestern Philippines. The aim was to compare between the two sites: (1) abiotic factors and zooplankton community parameters, and (2) the trophic structure using C and N stable isotopes. Low water (98–119?mV) and sediment (–121 to ?138?mV) Oxidation Reduction Potential values indicated a reducing environment in the impacted site. Zooplankton in the impacted site showed the typical community response to eutrophication (low diversity, but high total abundance due to the dominance of the cyclopoid copepod Oithona oculata), generally few elevated δ¹⁵N values, but a significant shift towards depleted ¹³C due to the organic enrichment of fish-farm feeds. Apart from suggesting a highly complex food web with POM and zooplankton as main food sources in the unimpacted site, the Bayesian mixing model simulation generated reduced complexity in feeding interactions between basal sources, zooplankton, and fish including adults of a key fish species, Siganus fuscescens, in the impacted sites. In this study, C and N stable isotope analysis has clarified the importance of zooplankton as fish prey in a seagrass bed food web.     

Using stable isotopes to reveal shifts in prey consumption by generalist predators.

The effectiveness of generalist predators in biological control may be diminished if increased availability of alternative prey causes individual predators to decrease their consumption of crop pests. Farming practices that enhance densities of microbidetritivores in the detrital food web can lead to increased densities of generalist predators that feed on pest species. The ability to predict the net biocontrol impact of increased predator densities depends upon knowing the extent to which individual predators may shift to detrital prey and feed less on crop pests when prey of the detritus-based food web are more abundant. We addressed this question by comparing ratios of stable isotopes of carbon (delta13C) and nitrogen (delta15N) in generalist ground predators and two types of prey (crop pests and microbidetritivores) in replicated 8 x 8 m cucurbit gardens subjected to one of two treatments: a detrital subsidy or no addition of detritus (control). Small sheet-web spiders (Linyphiidae) and small wolf spiders (Lycosidae) had delta13C values similar to those of Collembola in both the detrital and control treatments, indicating that small spiders belong primarily to the detrital food web. In control plots the larger generalist predators had delta13C values similar to those of the major insect pests, consistent with their known effectiveness as biocontrol agents. Adding detritus may have caused delta13C of one species of large wolf spider to shift toward that of the microbi-detritivores, although evidence is equivocal. In contrast, another large wolf spider displayed no shift in delta13C in the detrital treatment. Thus, stable isotopes revealed which generalist predators will likely continue to feed on pest species in the presence of greater densities of alternative prey.     

Energy transformations in two tropical Oxbow lakes

The flow and pattern of energy utilization in two tropical ecosystems where polyculture of fishes was practiced were studied. The rates of organic production made by the two tropical natural aquatic systems (oxbow lakes) were studied for two annual cycles. The study of utilization of energy in two tropical lakes revealed that herbivores were more efficient in energy utilization than autotrophs. The energy available in these systems was supplied by allochthonous source. Silver carp and grass carp were found to be most efficient in conversion of energy in the grazing food chain of the lake. The efficiency of the system to fish production was found to be 0.06 (OL-I) ?0.05% (OL-II) of the total available solar radiation. Various forms of life are all accompanied by energy changes, in the studied lakes were found to form a food web.     

Carbon-isotope ratio gradients in western arctic zooplankton

Zooplankton from 87 stations in the Bering, Chukchi and Beaufort Seas sampled in 1985, 1986 and 1987 showed a geographic gradient in stable carbon-isotope ratios (¹³C). The zooplankton most depleted in ¹³C were found in the central and eastern Beaufort Sea and those most enriched were from the Bering and Chukchi Seas. Average ¹³C values ranged from-20.9 to-26.7 for copepods and from-19.4 to-25.1 for euphausiids. Euphausiids show a minimum of 1.0 enrichment relative to copepods throughout the study area. Relative biomasses of the major zooplankton taxa varied significantly across the Alaskan Beaufort Sea in October 1986, with euphausiids dominating in the west and copepods in the east. These differences in taxonomic composition affected the weighted ¹³C values for total zooplankton and may produce an even more pronounced geographic gradient in zooplankton ¹³C than that found within a single taxon. The bowhead whale Balaena mysticetus migrates between wintering areas in the Bering Sea and summering areas in the Beaufort Sea and feeds over this geographic range. The zooplankton ¹³C gradient is the probable source of ¹³C oscillations found along the baleen plates of this planktivore.     

Daily photosynthesis,respiration, and carbon budgets in a tropical marine jellyfish (Mastigias sp.)

From measured diel photosynthesis and respiration rates, using oxygen electrodes, estimates of carbon flux between symbiotic algae (zooxanthellae) and host animal are presented for the marine scyphomedusan Mastigias sp. from a marine lake in Palau, Western Caroline Islands, during February and March 1982. The carbon budgets calculated for these lake medusae indicate that carbon fixed photosynthetically by zooxanthellae and made available to the host may satisfy up to 100% of the host's daily metabolic carbon demand (CZAR). The stable carbon isotope (¹³C) signature of the mesogleal carbon of lake Mastigias sp. was close to that of the zooxanthellae, supporting the interpretation that while these medusae may feed holozoically, some of their carbon comes from their symbionts. The diel photosynthesis, respiration, and preliminary estimates of carbon budgets of three individuals of another ecotype of Mastigias sp. collected from nearby oceanic lagoons are also given. Photosynthesis of lagoon medusae was generally greater than that for lake medusae of similar size, and lagoon medusae were phototrophic with respect to carbon, with commensurately greater CZAR values. Carbon translocated from the symbiotic algae also may contribute to the growth requirements of both lake and lagoon medusae. From carbon flux data, the lake jellyfish were estimated to contribute about 16% to the total primary productivity of their marine lake habitat.