Ratio Of Seston Research Articles

Complete exhaustion of dissolved nutrients in a large lowland river

Riverine phytoplankton takes up phosphate, dissolved silicate, and nitrate. We investigated which nutrients are depleted during a Lagrangian sampling in the free-flowing part of the River Elbe in 2023. As part of this study, we tested the hypotheses that nutrient depletion might be caused by (1) above-average phytoplankton biomass or by (2) decreased nutrient load of the river during previous years. Phytoplankton biomass increased up to 350 km in rivers and stopped increasing exactly when soluble reactive phosphorus had been completely consumed, and molar carbon to phosphorus ratios of seston indicated the beginning phosphorus limitation. The concentrations of dissolved silicate and nitrate dropped below the detection limit as well. In contrast to the results from eight previous longitudinal samplings taken in 2018–2022, nitrate exhaustion was detected for the first time in 2023 within the transect. This was caused neither by an above-average phytoplankton biomass nor by a declined overall nutrient load of the river in 2018–2023. Instead, denitrification appears to be the most plausible explanation for the downstream decrease of nitrate and the loss of total nitrogen which was supported by enrichment of nitrate stable isotopes and a decreasing ratio of nitrate 15N/18O.

Stoichiometry and planktonic communities structure in littoral and pelagic zones of two lakes with different trophic types in Belarus

Stoichiometric C : N : P ratios were compared between primary producers in littoral and pelagic ecosystems of mesotrophic relatively shallow lake Obsterno and shallow macrophyte covered low trophic lake Nobisto from May to October over the next two years. Elemental seston ratios of lake Obsterno revealed smaller differences between littoral and pelagic zones in comparison with lake Nobisto in 2017. During the studied period, in the both lakes, the seston C : N and C : P ratios were higher than the Redfield ratio (106 : 16 : 1 C : N : P) on most dates and N : P was always more than 16. Pelagic C : P and N : P ratios in lake Obsterno were the highest in May in 2017, July and September in 2018 with significant differences between littoral and pelagic zones. N : P ratios decreased in October but there were no significant differences among habitats. In lake Nobisto in 2018, seston C : P and N : P ratios increased from May to July in littoral and pelagic zones but then decreased in September to October. Our research shows differences in stoichiometric ratios in littoral and pelagic zones of these two lakes, which indicates food quality (seston C : N : P ratios) differences for zooplankton species depending on season and location.

Browning-induced changes in trophic functioning of planktonic food webs in temperate and boreal lakes: insights from fatty acids

The effects of lake browning on trophic functioning of planktonic food webs are not fully understood. We studied the effects of browning on the response patterns of polyunsaturated fatty acids and n−3/n−6 ratio in seston and compared them between boreal and temperate lakes. We also compared the regional differences and the effects of lake browning on the reliance of zooplankton on heterotrophic microbial pathways and the mass fractions of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in zooplankton. Lake browning was associated with increasing phytoplankton biomass and concentrations of EPA and DHA in both temperate and boreal lakes, but the seston n−3/n−6 ratio was lower in temperate than boreal lakes, most likely due the differences in phytoplankton community composition. The browning-induced increase in phytoplankton biomass was associated with increased reliance of zooplankton on a heterotrophic microbial pathway for both cladocerans and copepods in boreal and temperate lakes. This increased reliance on the heterotrophic microbial diet was correlated with a decrease in the EPA and DHA mass fractions in temperate copepods and a decrease in the n−3/n−6 ratio in boreal cladocerans and copepods. Our results indicate that although phytoplankton responses to lake browning were similar across regions, this did not directly cascade to the next trophic level, where zooplankton responses were highly taxa- and region-specific. These results indicate that lake browning should be considered as an overarching moderator that is linked to, e.g., nutrient increases, which have more immediate consequences on trophic interactions at the phytoplankton–zooplankton interface.

Impacts of invasive mussels on a large lake: Direct evidence from in situ control-volume experiments

Invasive dreissenid mussels have reengineered many freshwater ecosystems in North America and Europe. However, few studies have directly linked their filter feeding activity with ecological effects except in laboratory tests or small-scale field enclosures. We investigated in situ grazing on lake seston by dreissenid mussels (mainly quagga mussel Dreissena rostriformis bugensis) using a ‘control volume’ approach in the nearshore of eastern Lake Erie in 2016. Flow conditions were measured using an acoustic Doppler current profiler, surrounded by three vertical sampling stations that were arranged in a triangular configuration to collect time-integrated water samples from five different depths. Seston variables, including chlorophyll a, phaeopigment, particulate organic carbon and nitrogen, and particulate phosphorus, along with stoichiometric ratios and water flow over mussel colonies, were considered when estimating grazing rates. We observed suboptimal flow velocity for mussel grazing, i.e., 0.028 m s−1 at 0.1 m above bottom (mab), and resuspension was deemed minimal. Water temperature (mean: 25.1 °C) and an unstratified water column were optimal for grazing. Concentration of seston was low (mean: 0.2 mg L−1 particulate organic carbon) and decreased from surface to lakebed where noticeable depletion was observed. Grazing rates calculated at discrete depths varied substantially among trials, with maximum rates occurring at 0.25 or 0.5 mab. Positive grazing rates were restricted to 0.5 mab and below, defining an effective grazing zone (0.1–0.5 mab) in which the flow velocity, seston concentration, and water depth were consistently and positively correlated with grazing rates of different lake seston variables. Horizontal changes in stoichiometric ratios of seston were strongly associated with grazing rates, revealing higher uptake of particulate phosphorus than nitrogen and carbon. Our study supports the nearshore phosphorus shunt hypothesis, which posits that dreissenid mussels retain phosphorus on the lake bottom and contribute to a wide range of ecological effects on freshwater ecosystems.

Spatiotemporal carbon, nitrogen and phosphorus stoichiometry in planktonic food web in a northern coastal area

The concentrations of ambient nutrients and dissolved organic carbon (DOC) in northern coastal ecosystems often show large variations, due to the spatiotemporal differences in terrestial inputs. How these variations affect the stoichiometry of coastal planktonic organisms is, however, poorly known. Here we assessed the spatiotemporal variability of C, nitrogen (N), and phosphorous (P) concentrations of the seawater on the elemental stoichiometry of seston and dominant mesozooplankton taxa in a coastal area of the northern Baltic Sea. The freshwater inflow peaked in spring following the snowmelt and brought a significant amount of DOC, but not N and P to the coastal system. DOC was the main environmental descriptor for seston C:N stoichiometry. The C:N ratio of seston from 0.7 to 50 μm and mesozooplankton followed the temporal pattern of water C:N ratio, while the temporal trend of bacteria C:N showed an opposite pattern. Our results also indicated that the C:N ratio of seawater controlled both seston and mesozooplankton C:N ratios. Our findings imply that inflows of terrestrial DOC alter the stoichiometry and reduce the nutritional quality of planktonic food webs in northern coastal ecosystems.

High irradiation and low discharge promote the dominant role of phytoplankton in riverine nutrient dynamics

AbstractRivers play a relevant role in the nutrient turnover during the transport from land to ocean. Here, highly dynamic planktonic processes are more important compared to streams making it necessary to link the dynamics of nutrient turnover to control mechanisms of phytoplankton. We investigated the basic conditions leading to high phytoplankton biomass and corresponding nutrient dynamics in eutrophic, 8th order River Elbe (Germany). In a first step, we performed six Lagrangian sampling campaigns in the lower river section at different hydrological conditions. While nutrient concentrations remained high at low algal densities in autumn and at moderate discharge in summer, high algal concentrations occurred at low discharge in summer. Under these conditions, concentrations of silica and nitrate decreased and rates of nitrate assimilation were high. Soluble reactive phosphorus was depleted and particulate phosphorus increased inversely. Rising molar C:P ratios of seston indicated a phosphorus limitation of phytoplankton, so far rarely observed in eutrophic large rivers. Global radiation combined with mixing depth had a strong predictive power to explain maximum chlorophyll concentration. In a second step, we estimated nutrient turnover exemplarily for N during the campaign with the lowest discharge based on mass balances and metabolism‐based process measurements. Mass balance calculations revealed a total nitrate uptake of 423 mg N m−2d−1. Increasing phytoplankton density dominantly explained whole river gross primary production and related assimilatory nutrient uptake. In conclusion, riverine nutrient uptake strongly depends on the growth conditions for phytoplankton, which are favored at high irradiation and low discharge.

Factors controlling the stable isotope composition and C:N ratio of seston and periphyton in shallow lake mesocosms with contrasting nutrient loadings and temperatures

Summary Carbon (C) and nitrogen (N) stable isotope composition (15N:14N, δ15N and 13C:12C, δ13C) have been widely used to elucidate changes in aquatic ecosystem dynamics created by eutrophication and climate warming, often, however, without accounting for seasonal variation. Here, we aim to determine the factors controlling the stable isotope composition and C:N ratio of seston and periphyton in shallow lakes with contrasting nutrient loadings and climate; for this purpose, we followed the monthly stable isotope composition (c. 1 year) of seston (SES) and periphyton (PER) in 24 mesocosms mimicking shallow lakes with two nutrient treatments (enriched and unenriched) and three temperature scenarios (ambient, +3 and +5°C). Nutrient enrichment and warming had a stronger impact on the δ15N and δ13C values of seston than on periphyton, and the temporal isotopic variability in both communities was large. δ15NPER did not differ markedly between nutrient treatments, whereas δ15NSES was lower in the enriched mesocosms, possibly reflecting higher N2‐fixation by cyanobacteria. δ15NSES was higher in winter in the heated mesocosms and its dynamics was linked with that of NH4‐N, whereas δ15NPER showed a stronger association with NO3‐N. δ15NSES demonstrated a positive relationship with mean monthly temperature, indicating less isotope fractionation among autotrophs when production increased. δ13CSES was lowest in the enriched mesocosms during winter, whereas δ13CPER did not differ between nutrient treatments. δ13CSES and δ13CPER were positively related to pH, likely reflecting a pH‐induced differential access to dissolved carbon species in the primary producers. The positive δ13C‐temperature relationship suggested less fractionation of CO2 and HCO3− and/or larger use of HCO3− at higher temperatures. The C:N ratios varied seasonally and the differences between the enriched and unenriched mesocosms were stronger for seston than for periphyton. Particularly, the C:NSES ratios did not indicate deficiencies in N as opposed to the C:NPER ratios, supporting the observed changes in δ15N and suggesting that seston and periphyton have access to different sources of nutrients. We did not observe any clear effect of temperature warming on the C:N ratios. Our study provides evidence of strong seasonality in the isotopic composition and C:N ratios of seston and periphyton across nutrient and temperature levels; also, we identified several factors that are likely to modulate the strength and variability in stable isotopes values and stoichiometry of sestonic and periphytic communities under these scenarios.

Molecular and ecological characterization of toxic cyanobacteria from the Bay of Quinte (Lake Ontario) and Maumee Bay (Lake Erie)

Abstract Two Great Lake embayments, Bay of Quinte (Lake Ontario) and Maumee Bay (Lake Erie) prone to toxic cyanobacteria blooms were sampled in early and late summer in 2006 to determine which cyanobacteria genotypes were present and how many of those were capable of producing microcystins. Microcystins, chlorophyll a, soluble reactive phosphorus and stoichiometric seston ratios of carbon, nitrogen and phosphorus were measured on the same samples to provide an ecological characterization of the phytoplankton. The V3 hypervariable region of the 16S rRNA and aminotransferase (AMT) genes, microcystin (mcyE) and nodularin (nda) were analyzed with PCR-DGGE and quantitative real-time PCR to describe cyanobacterial communities. DGGE results indicated species capable of producing microcystins were present at some point in all study sites and were most likely from species within the genus Microcystis. Concentrations of microcystins were greatest when hepatotoxic cell numbers were highest and when seston stoichiometry indicated cells were not nutrient stressed. This study demonstrated the effective use of a combination of molecular and ecological monitoring of cyanobacterial communities in regions that are susceptible to toxic cyanobacteria blooms.

Availability and utilization of waste fish feed by mussels Mytilus edulis in a commercial integrated multi-trophic aquaculture (IMTA) system: A multi-indicator assessment approach

Fish feed waste enhancement of the particulate food supply and performance of mussels Mytilus edulis suspended near salmon cages at an integrated multi-trophic aquaculture (IMTA) site was assessed using a multi-indicator approach. Dietary indicators included bulk measurements of seston quantity and nutritional quality, proximate analysis (PA), fatty acid (FA) and stable isotope (SI) composition. Mussel tissue indicators consisted of PA and FA composition. Mussel performance was assessed from physiological integrations (scope for growth, SFG), growth efficiency (K2) and condition index (CI). All measurements were made over 2 days at a commercial IMTA farm and a monoculture mussel farm in the Bay of Fundy (Canada). Significant differences detected in seston quantity and quality were within the range of natural spatial variability. The SFG of IMTA mussels was lower (28.71Jh−1) than monoculture mussels (38.71Jh−1) and reflected site differences in natural food availability and composition that affected absorption rate. PA of mussel organs didn't reflect a significant fish feed contribution to the mussel diet. However, dietary enhancement and assimilation of fish feed waste was demonstrated by significantly higher levels of feed FA biomarkers 20:1ω9, 18:2ω6, 18:1ω9 and low ω3/ω6 ratio in seston, mussel tissues and feces at the IMTA site than at the mussel farm. SI (δ13C and δ15N) in seston and mussel feces significantly differed among sites and IMTA mussels had significantly higher CI (21%) than monoculture individuals (16%). It was concluded that bulk indicators of the diet, short-term physiological integrations, and PA of mussel tissues have a limited capacity to detect dietary enhancement at IMTA sites. FA and SI tracers of fish feed waste were shown to be more sensitive for detecting the low-levels of diet enhancement within the large range of natural seston variation.

Delivery of nutrients and seston from the Muskegon River Watershed to near shore Lake Michigan

Drowned river mouth lakes are major features of coastal Great Lakes habitats and may influence nutrient and organic matter contributions from watersheds to near shore coastal zones. In May through October 2003, we measured loads of nutrients, surficial sediment, and seston to track the delivery of riverine-derived materials from the lower Muskegon River Watershed (MRW) into the near shore area of southeast Lake Michigan. Nutrient flux data indicated that seasonal loads of 1800metrictons (MT) of particulate organic carbon, 3400MT of dissolved organic carbon, and 24MT of total phosphorus were discharged from the lower Muskegon River, with approximately 33% of TP load and 53% of the POC load intercepted within the drowned river mouth terminus, Muskegon Lake. Carbon: phosphorus molar ratios of seston in Muskegon River (C:P=187) and Muskegon Lake (C:P=176) were lower than in Lake Michigan (C:P=334), indicating phosphorus limitation of phytoplankton in near shore Lake Michigan. Isotopic signatures of seston collected in Muskegon Lake were depleted in δ13C (−30.8±1.6‰) relative to the isotope signatures of seston from Lake Michigan (−26.2±1.3‰) or the mouth of the Muskegon River (−28.1±0.5‰), likely due to the presence of biogenic methane in Muskegon Lake. Seston δ15N increased on a strong east-to-west gradient within Muskegon Lake, indicating significant microbial processing of nutrients. The extent of nutrient uptake in Muskegon Lake altered the chemical and isotopic characterization of seston flowing into Lake Michigan from Muskegon River.

Strong spatial differentiation of N and P deficiency, primary productivity and community composition between Nyanza Gulf and Lake Victoria (Kenya, East Africa) and the implications for nutrient management

Summary Study of phytoplankton nutrient status, biomass, productivity and species composition was carried out between March 2005 and March 2006, along a transect between north‐eastern open Lake Victoria and the large, shallow Nyanza Gulf in order to examine how the terrestrial run‐off can influence phytoplankton community and nutrient status and determine whether nutrient management of catchment run‐off has the potential to control the algal blooms in the gulf. Hydrological and nutrient differences between the open lake and the gulf create a transition from P deficiency for phytoplankton within the gulf to nitrogen deficiency in open lake. The shallow and turbid gulf was continuously dominated by non‐nitrogen‐fixing filamentous and chroococcale colonial cyanobacteria, but seasonal stratification and deeper mixing depth in the open lake favoured diazotrophic cyanobacteria and diatoms. Seston ratios and metabolic nutrient assays indicated the gulf to be sufficiently phosphorus deficient to impose P limitation on phytoplankton growth and biomass. In contrast, the open lake is not P deficient and is more likely to experience N deficiency that favours diazotrophic cyanobacteria. Because of high turbidity in the gulf, the euphotic zone is very shallow, limiting integral primary productivity compared to the less turbid open lake; high PAR extinction may also favour Microcystis blooms in the gulf. Increased P loading into the gulf may translate to higher algal biomass, mainly of the bloom‐forming and potentially toxic cyanobacteria, and therefore, reduction in P loading into the gulf should be a management priority. However, a review of historical data indicates that the greatest change in water quality in the gulf is increased turbidity that reduces light availability and may limit algal growth more than P deficiency in years of high rainfall and river discharge.

Seasonal variation in marine C:N:P stoichiometry: can the composition of seston explain stable Redfield ratios?

Abstract. Seston is suspended particulate organic matter, comprising a mixture of autotrophic, heterotrophic and detrital material. Despite variable proportions of these components, marine seston often exhibits relatively small deviations from the Redfield ratio (C:N:P = 106:16:1). Two time-series from the Norwegian shelf in Skagerrak are used to identify drivers of the seasonal variation in seston elemental ratios. An ordination identified water mass characteristics and bloom dynamics as the most important drivers for determining C:N, while changes in nutrient concentrations and biomass were most important for the C:P and N:P relationships. There is no standardized method for determining the functional composition of seston and the fractions of POC, PON and PP associated with phytoplankton, therefore any such information has to be obtained by indirect means. In this study, a generalized linear model was used to differentiate between the live autotrophic and non-autotrophic sestonic fractions, and for both stations the non-autotrophic fractions dominated with respective annual means of 76 and 55%. This regression model approach builds on assumptions (e.g. constant POC:Chl-a ratio) and the robustness of the estimates were explored with a bootstrap analysis. In addition the autotrophic percentage calculated from the statistical model was compared with estimated phytoplankton carbon, and the two independent estimates of autotrophic percentage were comparable with similar seasonal cycles. The estimated C:nutrient ratios of live autotrophs were, in general, lower than Redfield, while the non-autotrophic C:nutrient ratios were higher than the live autotrophic ratios and above, or close to, the Redfield ratio. This is due to preferential remineralization of nutrients, and the P content mainly governed the difference between the sestonic fractions. Despite the seasonal variability in seston composition and the generally low contribution of autotrophic biomass, the variation observed in the total seston ratios was low compared to the variation found in dissolved and particulate pools. Sestonic C:N:P ratios close to the Redfield ratios should not be used as an indicator of phytoplankton physiological state, but could instead reflect varying contributions of sestonic fractions that sum up to an elemental ratio close to Redfield.

Nitrogen and carbon isotope fractionations of zooplankton consumers in ponds: potential effects of seston C:N stoichiometry

C : N stoichiometry in food sources is known to influence nitrogen isotope fractionation of consumers, but the relationship between C : N stoichiometry and isotope fractionation has never been tested in the field. In this study, we investigated the effects of the C : N ratio of food sources on nitrogen and carbon isotope fractionation (Δδ15N and Δδ13C) of consumers, using zooplankton communities in 15 ponds in Japan. The fractionation variations we found contradicted the assumption of a 3.4‰ enrichment in δ15N of consumers per trophic level. However, the negative relationships between Δδ15N of filter-feeding zooplankton and the C : N ratio of seston supported the isotope homeostasis hypothesis, which predicts isotopic homeostasis of consumers to vary with respect to their body nutrient composition. The Δδ15N of Eodiaptomus sp. and cyclopoid copepods did not correlate with the C : N ratio of the seston. Further studies should pay attention to the isotope fractionation of consumers in the field, especially given the increased use of isotope fractionation to elucidate the structure of natural food webs.

Seasonal growth variation in Chlamys islandica (Bivalvia) from sub-Arctic Greenland is linked to food availability and temperature

In order to assess the role of different environmental parameters in the population dynamics of marine invertebrates in the Arctic, we examined seasonal variation in individual bio- mass, shell growth, and mass production of somatic and reproductive tissues of immature and matur- ing scallops Chlamys islandica suspended in culture nets at 15 and 30 m depth in SW Greenland from May 2007 to October 2008. All 3 parameters varied both seasonally and between depths. Individual shell growth rate and biomass were poor proxies for the actual mass growth rate on a seasonal scale. Minimum and maximum mass growth rates were observed from November to February and in April-May, respectively. Simultaneous monitoring of biotic and abiotic parameters in the water col- umn made it possible to establish a growth model relating variation in mass growth rate to variation in environmental conditions. The best-fit model (R 2 = 0.71) indicated that total mass growth rate depended on chlorophyll a concentration, carbon-to-nitrogen ratio of seston, and water temperature. While availability of high-quality food items affected growth positively, the growth model indicated a negative effect of increasing temperature on the mass growth rate of C. islandica. These results indicate that scallops in SW Greenland are resource-limited and that elevated temperature through its effect on metabolic costs reduces growth efficiency. Hence, it is most likely that the growth capac- ity of C. islandica in SW Greenland is either never realized or only attained for short periods of time (hours to days) under the present conditions.

Seasonal variations in stable isotope ratios of two biomanipulation fishes and seston in a large pen culture in hypereutrophic Meiliang Bay, Lake Taihu

This paper reports on seasonal changes in stable carbon and nitrogen isotope ratios of seston and muscle tissue of silver carp and bighead carp during 2004 and 2005, focusing primarily on the carbon sources and trophic relationships among phytoplankton, zooplankton and silver carp and bighead carp in a large fish pen of Meiliang Bay (Lake Taihu, China). δ 13C showed a minimal value in March 2005 and a maximal value in August 2005 in seston both inside and outside the pen, whereas δ 15N of seston showed the minimum in winter and the maximum during algal blooms. A positive correlation between δ 13C of silver carp and that of seston suggested that temporal variation of δ 13C in seston was preserved in fish via the food chain. The differences of δ 13C among seston, zooplankton and muscle tissue of silver carp and bighead carp ranged only 0.2–1.7%, indicating that plankton production was the primary food source of filter-feeding fishes. According to a mass balance model, we estimated that the contributions of zooplankton to the diets of silver carp and bighead carp were 45.7% and 54.3%, respectively, based on the δ 15N values of zooplankton and planktivorous fishes.

Nitrogen dynamics at the sediment–water interface in shallow, sub-tropical Florida Bay: why denitrification efficiency may decrease with increased eutrophication

Nitrogen (N) dynamics at the sediment–water interface were examined in four regions of Florida Bay to provide mechanistic information on the fate and effects of increased N inputs to shallow, subtropical, coastal environments. Dissimilatory nitrate (NO3−) reduction to ammonium (DNRA) was hypothesized to be a significant mechanism retaining bioreactive N in this warm, saline coastal ecosystem. Nitrogen dynamics, phosphorus (P) fluxes, and sediment oxygen demand (SOD) were measured in north-central (Rankin Key; eutrophic), north-eastern (Duck Key; high N to P seston ratios), north-western (Murray Key; low N to P ratios), and central (Rabbit Key; typical central site) Florida Bay in August 2004, January 2005, and November 2006. Site water was passed over intact sediment cores, and changes in oxygen (O2), phosphate (o-PO43−), ammonium (NH4+), NO3−, nitrite (NO2−), and N2 concentrations were measured, without and with addition of excess 15NO3− or 15NH4+ to inflow water. These incubations provided estimates of SOD, nutrient fluxes, N2 production, and potential DNRA rates. Denitrification rates were lowest in summer, when SOD was highest. DNRA rates and NH4+ fluxes were high in summer at the eutrophic Rankin site, when denitrification rates were low and almost no N2 came from added 15NO3−. Highest 15NH4+ accumulation, resulting from DNRA, occurred at Rabbit Key during a picocyanobacteria bloom in November. 15NH4+ accumulation rates among the stations correlated with SOD in August and January, but not in November during the algal bloom. These mechanistic results help explain why bioreactive N supply rates are sometimes high in Florida Bay and why denitrification efficiency may decrease with increased NO3− inputs in sub-tropical coastal environments.

Effects of the zebra mussel, an exotic freshwater species, on seston stoichiometry

We examined the effect of the zebra mussel, Dreissena polymorpha, an exotic species, on seston stoichiometry by conducting laboratory experiments in which we varied nutrient composition of seston and mussels over time. Zebra mussels altered the stoichiometry of seston through removal of particulate organic nutrients and changed the stoichiometry of the dissolved nutrient pool through nutrient excretion. Grazers had stronger effects on carbon : phosphorus (C : P) and nitrogen (N) : P ratios than on the C:N ratio of seston. Elemental residence time in tissue and high mass‐specific nutrient excretion by small mussels caused small mussels to be more efficient nutrient recyclers than larger mussels. Zebra mussels reduced P availability through enhancing C: P and N: P molar ratios of seston during the period extending from June to August, when P was limited in the lake, and increased the C:N molar ratio of seston in June, when N was at the minimum level in the lake. C: P and N: P molar ratios for zebra mussel tissue were higher in August and somewhat in September than in all other months. N was retained more efficiently than P in Dreissena tissue. Nutrient mass‐specific uptake rate was higher than excretion rate by zebra mussels.

Re-oligotrophication by phosphorus reduction and effects on seston quality in lakes

We review published data of re-oligotrophication studies in different types of lakes and discuss whether re-oligotrophication by phosphorus reduction measures can affect seston quality in lakes due to an increase in C:P ratios of food for zooplankton. We analyze whether such an increase in elemental ratio may give rise to a stoichiometric constraint or bottleneck for regulation of phytoplankton by zooplankton and thus cause a decrease in trophic transfer efficiency. Because seston C:P ratios of seston tend to decrease with increasing P concentration in lakes, i.e. by increasing eutrophication, the reverse may be true, i.e. the C:P ratio of the seston will increase when lakes become re-oligotrophicated by P reduction. This may imply lower food quality for P-demanding seston grazers, like Daphnia, which have both relatively low C:P ratios and are quite homeostatic with regard to C:P ratios. Consequently, the reduced grazer biomass may lead to lower grazing pressure on the phytoplankton and may result in lower trophic transfer efficiency. On the other hand, oligotrophication commonly leads to improved food quality in terms of composition of the phytoplankton community, e.g. increase of mixotrophs and reduced share of cyanobacteria. These two qualitative traits of seston (higher C:P ratios and change in phytoplankton composition) may thus work in opposite directions during re-oligotrophication. Most investigated lakes – both deep and shallow – responded by improvement in underwater light climate due to reduction of P loadings. Several compensatory mechanisms prevented the occurrence of a stoichiometric bottleneck for top-down control. From a few documented cases of both deep and shallow lakes, where seston stoichiometry, the presence of Daphnia and the trophic transfer efficiency were concurrently measured during the process of re-oligotrophication, we may conclude that the anticipated increase in C:P ratios of the seston is not straightforward. It depends on lake-specific properties such as morphometry, water residence time, water temperature and trophic structure. A stoichiometric bottleneck is only likely to occur during summer in (1) the epilimnion of small deep and stratified lakes with a long water residence time and low mixing regime, and (2) shallow lakes with high C:P ratios caused by a relatively high detritus content of seston and/or grazing resistant algae, like colony forming cyanobacteria.

Stoichiometric relationships in vernal pond plankton communities

Summary1. The light‐nutrient hypothesis (LNH) predicts that changes in light supply can alter the balance of nutrient and energy limitation in primary producers. We tested this prediction by examining temporal changes in vernal forest ponds, which are highly dynamic systems with respect to seasonal change in light and nutrient supply. In three vernal ponds that differ in productivity, we measured changes in light, total and seston nitrogen and phosphorus, and seston carbon and chlorophyll during the spring, before and after tree leaf‐out. We also quantified changes in the population dynamics of the major zooplankton grazers in these systems.2. In each pond, nutrient levels increased and light levels declined, creating a temporal shift in light‐nutrient supply to the plankton. Results generally supported predictions of stoichiometric theory and the LNH, but there were notable exceptions.3. Seston C : N : P ratios rapidly changed in response to dramatic increases in N and P supply rates. However, seston N : P was typically lower than values for total N : P in the water. Furthermore, as predicted, we observed a decline in seston C : P as the light : nutrient ratio declined, but seston C : N simultaneously increased. These results suggest an unexpected shift towards potential nitrogen limitation. Alternatively, this change in nutrient ratios may be driven by a seasonal change in phytoplankton composition or nutritional mode.4. Seston carbon concentrations remained stable despite seasonal changes in grazing intensity associated with the phenology of large‐bodied Daphnia grazers. However, chlorophyll concentrations declined dramatically as the season progressed, resulting in a simultaneous decline in the C : Chlorophyll ratio of seston. Both pond shading and increased grazing probably contributed to the decline in chlorophyll.

Influence of biochemical, mineral and morphological features of natural food on tropical cladocerans

Five laboratory experiments were performed to evaluate the effect of supplements of fatty acids and a green alga on the individual growth and reproduction of three species of tropical cladocerans Ceriodaphnia richardi, Daphnia ambigua, and D. gessneri feeding on natural seston from the Brazilian Lake Monte Alegre. Cohorts of newborns from cultivated females were submitted to one of the following treatments: (1) Natural seston, (2) Natural seston + microcapsules of EPA and DHA or linoleic and linolenic fatty acids, (3) Natural seston + oil-free microcapsules, and (4) Natural seston + green alga Scenedesmus spinosus (1 mg C l−1). Particulate organic carbon, algal carbon, C:P ratios of seston and green alga, polyunsaturated fatty acids (PUFA) content of seston and cladocerans, as well as phytoplankton composition, size, and shape were measured. The addition of fatty acids to seston did not significantly enhance growth and reproduction of the cladocerans, suggesting that sestonic PUFA content is sufficient for promoting cladoceran development, even in the cool–dry season when the fatty acids used in the experiments were 5–10 times lower in the seston than in the warm–wet season. Despite high C:P molar ratios in most experiments, there was only one indication of growth limitation by P. Reproduction was more affected than individual growth on some occasions by food quantity (energy) caused apparently by algal size, morphology, and digestion resistance. Energy availability, which is affected by algae morphological characteristics, seems to prevail over PUFA and P in controlling growth and reproduction of cladocerans in tropical Lake Monte Alegre.