Dissolved Organic Carbon Gradient Research Articles

Effects of dissolved organic carbon gradient on epilimnetic zooplankton communities in lakes

Lake browning is expected to change aquatic ecosystems considerably. The changes that may occur in zooplankton communities with a high concentration of dissolved organic carbon (DOC) are little studied and not yet sufficiently understood. We studied zooplankton communities in Finland in 27 lakes with a wide DOC range. We explored how zooplankton diversity and biomass vary along a DOC gradient and how species belonging to different functional groups respond to varying DOC concentrations. The total biomass of zooplankton was not related to DOC concentration, but diversity decreased and the negative linear trend in diversity coincided with an increase in omnivorous zooplankton taxa, whereas several other herbivorous zooplankton taxa were displaced from the community along the gradient of DOC. The results of our study suggest that some well-adapted taxa, especially omnivorous taxa, can benefit from lake browning, but a larger number of taxa suffer from unfavourable conditions caused by high DOC concentration. DOC-induced changes in predation pressure and changes in phytoplankton community in terms of resource availability for zooplankton should be emphasized in future research to understand the effects of lake browning on zooplankton.

Factors behind the threshold‐like changes in lake ecosystems along a water colour gradient: The effects of dissolved organic carbon and iron on euphotic depth, mixing depth and phytoplankton biomass

Abstract The effects of water colour, dissolved organic carbon (DOC), iron (Fe) and fetch on the mixing depth (Zmix), euphotic depth (Zeu) and phytoplankton biomass of lakes were studied to clarify the factors behind changes in lake productivity along a DOC gradient. The study was conducted by field sampling 102 lakes in Finland. The lakes were situated mainly in forested areas. At DOC concentrations below 15 mg/L, light attenuation was explained both by Fe and DOC, but at higher DOC concentrations the effect of Fe faded. For the absorption of different wavelengths of photosynthetic light, both DOC and Fe had a significant effect up to 475 nm, but for longer wavelengths only the effect of DOC was significant. Both Zmix and Zeu decreased logarithmically with increasing DOC and water colour. Water colour was determined by DOC and Fe. Zmix increased along with increasing fetch. Zmix exceeded Zeu when DOC concentration was >14.4 mg/L and water colour >106 mg Pt/L (platinum as chloroplatinate ion 3). Above the 14.4 mg/L DOC threshold, the illuminated part of the mixed layer decreased logarithmically with increasing water colour, leaving an increasing part of the mixed epilimnion below the photosynthetic layer. Chlorophyll‐a concentration showed a unimodal relationship with DOC, with the highest biomass occurring at DOC concentrations of 15.7–15.9 mg/L. The results suggested that above DOC concentration 14 mg/L and water colour 106 mg Pt/L, both quality and quantity of light in the epilimnion restrict production by phytoplankton. This can explain the observed threshold‐like changes in lake productivity along a DOC gradient.

Differential responses for stream algal assemblages exposed to factorial N and P enrichment along an in situ DOC gradient

Nitrogen (N) and phosphorus (P) enrichment from the surrounding landscape can enhance the growth and abundance of algal assemblages, both suspended in the water column and occurring as benthic biofilms in the receiving waters of resident stream ecosystems. However, it is unclear how enhanced levels of dissolved organic carbon (DOC) might mediate the response of algae to nutrients. In this study, we aim to understand the relationship between N and P enrichment in stream reaches with varying levels of DOC within a common watershed. Both suspended and benthic algal assemblages were exposed to increasing (low, moderate, high) levels of N and P in an in situ 3 × 3 factorial experimental design. Experiments were carried out simultaneously at three sites of varying DOC concentration, the results of which, showed that both assemblages responded very differently to enrichment. For suspended algae, biomass was enhanced following enrichment with both N and P singularly and in combination, particularly at the highest concentrations levels for both nutrients (3-way ANOVA interactions; p < 0.05). This pattern was consistent at all levels of DOC at the three sites (3-way ANOVA Nutrient-site interaction; F = 1.4, p = 0.228). In contrast, benthic algal biomass progressively increased with enhanced concentration of both N and P, but only at the lowest level of DOC (3-way ANOVA site term; F = 84.2, p = 0.0001). No appreciable biomass accumulation was observed under moderate or high DOC conditions and this pattern was likely attributed to low light penetration at these two sites (ANOVA). Our results indicate that eutrophication is likely to manifest itself in a complex manner throughout watersheds where loading of N and P interacts with DOC.

Crustacean zooplankton densities in northern temperate lakes are related to habitat temperature across a wide gradient in lake dissolved organic carbon and nutrient content

Abstract Lake crustacean zooplankton densities often are negatively correlated with terrestrial dissolved organic carbon (DOC) concentrations. These reductions in zooplankton with increased DOC are hypothesised to be linked to diminished resource quantity or lower resource quality as terrestrial material is low in essential nutrients and macromolecules. The impact of DOC on lake physics also potentially reduces available habitat for zooplankton as the warm and well‐oxygenated epilimnion is shallower in lakes with high DOC concentrations. Our goal was to investigate these potential mechanisms to determine the influence of DOC on drivers of zooplankton densities in a survey of north temperate lakes. We sampled crustacean zooplankton densities in 10 lakes that varied in mean DOC concentration from 6 to 27 mg L−1. We also measured resource availability as chlorophyll concentration, resource quality as essential fatty acid (EFA) concentration and the stoichiometric ratio of carbon‐to‐phosphorus (C:P), and habitat availability as integrated habitat temperature and dissolved oxygen to determine the strongest predictor of zooplankton densities across lakes. In addition, we quantified zooplankton habitat use through Schindler trap profiles through the water column. Zooplankton densities were most strongly related to integrated habitat temperature and were not closely related to measures of resource quantity or quality. Depth of the mix layer was negatively correlated with DOC concentration, yet there was no relationship between DOC concentration and zooplankton habitat use. Overall resource quantity and quality increased across the DOC gradient, as chlorophyll and EFA concentration were greatest in lakes with higher DOC. Our results indicate the potential for physics‐mediated responses between lake DOC and zooplankton density. As lakes with greater DOC concentrations have, on average, shallower mixed layers and colder habitats, zooplankton may either be constrained to a relatively smaller proportion of the catchment or experience reduced temperatures that may delay development or feeding rates. Lake DOC concentrations are projected to increase under future climate scenarios, so accompanying changes in lake temperature profiles are likely to follow. These also may reduce food‐web productivity, as shrinking mixed layers or colder total water columns resulting from steeper thermoclines may reduce habitat availability or suitability for zooplankton communities.

Dissolved organic carbon affects the occurrence of deep chlorophyll peaks and zooplankton resource use and biomass

Abstract Concentrations of dissolved organic carbon (DOC) are increasing in many parts of the world and vary considerably among lakes. As a result, it is important to understand the effects of DOC on zooplankton, which are a key component of freshwater food webs. Deep chlorophyll maxima (DCMs) also are common in many lakes and numerous studies suggest that they may be an important resource for zooplankton. In a survey of eight boreal lakes spanning a gradient of DOC (3.5–9.2 mg/L), we assessed variations in the occurrence of DCMs, zooplankton biomass and their use of basal resources using stable isotopes of carbon (C), nitrogen (N) and hydrogen (H). DCMs occurred only in lakes with lower DOC concentrations and Bayesian stable isotope mixing models indicated that they contributed between 22% and 72% of the assimilated diet of zooplankton. Food quality as indicated by C:P and chlorophyll a: C ratios increased with depth and in DCMs. The proportion of C ultimately derived by zooplankton from terrestrial sources increased from 6% to 27% with DOC. Zooplankton biomass among lakes declined with increasing DOC concentrations, the absence of DCMs and with increasing allochthony. Our results suggest that increasing inputs of DOC may suppress zooplankton in many lakes by affecting both resource quality and the availability of metalimnetic phytoplankton resources. Because zooplankton occupy a central position in freshwater food webs, declines in their biomass may further affect phytoplankton dynamics and fish productivity.

Sensitivity of Steady State, Deep Ocean Dissolved Organic Carbon to Surface Boundary Conditions

AbstractWe use the transport matrices of a data‐constrained circulation model to efficiently compute the steady state distribution of the deep ocean dissolved organic carbon (DOC) at a 1° horizontal resolution by propagating the surface DOC boundary conditions into the ocean interior. An equivalent simulation in the traditional forward modeling approach would be prohibitively computationally expensive. Our model simulates the total DOC as the sum of two DOC pools, the refractory and the semi‐labile. The model is able to simulate the large‐scale features of the deep ocean DOC without local sources or sinks of DOC in the ocean interior. The deep ocean DOC in the model is sensitive to the preformed DOC concentrations in the formation sites of deep and bottom waters, where observations are lacking. Furthermore, our model experiments indicate that the deep Atlantic DOC gradient is sensitive to the mixing of deep waters with different concentrations of preformed refractory DOC, the transport of semi‐labile DOC from the surface North Atlantic, and the decay rate of semi‐labile DOC. These, combined with the observation that much of the deep ocean DOC gradient is in the Atlantic, suggests that the semi‐labile DOC may be an important component of the deep Atlantic DOC. Finally, we show that DOC export depends substantially on the depth level where it is evaluated.

Seasonality of Dissolved Organic Carbon Exchange Across the Strait of Gibraltar

AbstractExchange of mass, heat, and solutes across the Strait of Gibraltar is fundamental to understand the circulation and biogeochemistry of the Mediterranean. Here we focus on the exchange of dissolved organic carbon (DOC) using data from 12 surveys conducted between 2008 and 2015. DOC exchange exhibits a marked bimodal distribution with minima in late June and late October and maxima in mid‐April and late August. This pattern is mainly due to seasonal variation of the DOC gradient between the Atlantic Surface Water entering the Mediterranean and the deep opposite flow of Mediterranean Overflow Water. The gradient is controlled by the different seasonal cycles followed by the DOC in the two layers. Annual average DOC import from the Atlantic is equivalent to 4.2 ± 1.5 Tg C yr−1, which represents 53% of the external DOC inputs and contributes to support 32% of the net heterotrophy of the Mediterranean.

The microbial mercury link in oligotrophic lakes: Bioaccumulation by picocyanobacteria in natural gradients of dissolved organic matter

Andean Patagonian lakes are oligotrophic systems characterized by low dissolved organic carbon (DOC) levels and moderate to high Hg concentration that determine naturally high Hg/DOC ratios and bioavailability. In these lakes, microbial food webs are extremely important in Hg trophodynamics, being that the picophytoplankton fraction is a major entrance path of Hg2+ into pelagic food webs. This study analyzed the bioaccumulation of Hg2+ by the picocyanobacteria Synechococcus sp. using the radiotracer 197Hg2+ and water from four Andean Patagonian lakes presenting a natural gradient of DOM concentration and quality. Hg2+ bioaccumulation by Synechococcus was calculated as the uptake of Hg2+ per biovolume unit (volume concentration factor VCF; pL μm−3). Hg uptake showed a wide variation (13 < VCF< 300 pL μm−3) in the natural DOC gradient tested (0.7–4 mg L−1; Hg2+/DOC ratio: 1.8–14 ng mg−1). The bioaccumulation of Hg2+ in Synechococcus decreased exponentially with DOC concentration. Differences in the quality of dissolved organic matter (DOM) among lake water influenced also Hg2+ bioaccumulation. Naturally degraded DOM, with low molecular weight/size, promoted higher Hg uptakes in Synechococcus compared to humic DOM, rich in high molecular weight/size aromatic compounds, that retained Hg in the dissolved phase. In Andean Patagonian lakes picocyanobacteria are pivotal organisms in the Hg cycling, taking dissolved Hg2+ and transferring it to pelagic food webs, as well as fueling the benthic Hg pathway through sedimentation.

Changes in food web dynamics of low Arctic ponds with varying content of dissolved organic carbon

ABSTRACT An influx of terrestrial dissolved organic carbon (DOC) into freshwater habitats can regulate a range of ecosystem characteristics, from water clarity to productivity. To understand the extent to which DOC can regulate ecosystem functioning, we conducted a survey to determine the source of DOC in low Arctic ponds close to the Arctic Circle (Kangerlussuaq, Greenland), including its role in food web dynamics. We used a multiple element (carbon, nitrogen, and hydrogen) stable isotope approach to examine the proportional contribution of different sources to aquatic consumers in nine arctic ponds that spanned a broad gradient of DOC (6.6–60.1 mgL-1). Our results show that benthic and pelagic primary production decreased along a gradient of increasing DOC content. Additionally, the changes in the organic matter pool with increasing DOC translated into changes in consumer resource use. We found significant differences in resource use between species. All consumers relied on benthic autotrophic material when DOC was low; but when DOC was high some consumers changed their diet. Collectively, our findings demonstrate how the concentration of DOC influences aquatic production and our study can be used as a baseline to predict how the aquatic food web may respond to regionally changing DOC concentrations.

Nitrogen effects on the pelagic food web are modified by dissolved organic carbon

Global environmental change has altered the nitrogen (N) cycle and enhanced terrestrial dissolved organic carbon (DOC) loadings to northern boreal lakes. However, it is still unclear how enhanced N availability affects pelagic food web efficiency (FWE) and crustacean zooplankton growth in N limited boreal lakes. Here, we performed in situ mesocosm experiments in six unproductive boreal Swedish lakes, paired across a DOC gradient, with one lake in each pair fertilized with N (2011: reference year; 2012, 2013: impact years). We assessed how zooplankton growth and FWE were affected by changes in pelagic energy mobilization (PEM), food chain length (phytoplankton versus bacterial production based food chain, i.e. PP:BP), and food quality (seston stoichiometry) in response to N fertilization. Although PP, PEM and PP:BP increased in low and medium DOC lakes after N fertilization, consumer growth and FWE were reduced, especially at low DOC—potentially due to reduced phytoplankton food quality [increased C: phosphorus (P); N:P]. At high DOC, N fertilization caused modest increases in PP and PEM, with marginal changes in PP:BP and phytoplankton food quality, which, combined, led to a slight increase in zooplankton growth and FWE. Consequently, at low DOC (<12 mg L−1), increased N availability lowers FWE due to mismatches in food quality demand and supply, whereas at high DOC this mismatch does not occur, and zooplankton production and FWE may increase. We conclude that the lake DOC level is critical for predicting the effects of enhanced inorganic N availability on pelagic productivity in boreal lakes.

Life history constraints explain negative relationship between fish productivity and dissolved organic carbon in lakes.

Resource availability constrains the life history strategies available to organisms and may thereby limit population growth rates and productivity. We used this conceptual framework to explore the mechanisms driving recently reported negative relationships between fish productivity and dissolved organic carbon (DOC) concentrations in lakes. We studied populations of bluegill (Lepomis macrochirus) in a set of lakes with DOC concentrations ranging from 3 to 24 mg/L; previous work has demonstrated that primary and secondary productivity of food webs is negatively related to DOC concentration across this gradient. For each population, we quantified individual growth rate, age at maturity, age‐specific fecundity, maximum age, length‐weight and length‐egg size relationships, and other life history characteristics. We observed a strong negative relationship between maximum size and DOC concentration; for instance, fish reached masses of 150 to 260 g in low‐DOC lakes but <120 g in high‐DOC lakes. Relationships between fecundity and length, and between egg size and length, were constant across the DOC gradient. Because fish in high‐DOC lakes reached smaller sizes but had similar fecundity and egg size at a given size, their total lifetime fecundity was as much as two orders of magnitude lower than fish in low‐DOC lakes. High DOC concentrations appeared to constrain the range of bluegill life history strategies available; populations in high‐DOC lakes always had low initial growth rates and high ages at maturity, whereas populations in low‐DOC showed higher variability in these traits. This was also the case for the intrinsic rates of natural increase of these populations, which were always low at the high end of the DOC gradient. The potentially lower capacity for fish populations in high‐DOC lakes to recover from exploitation has clear implications for the sustainable management of recreational fisheries in the face of considerable spatial heterogeneity and ongoing temporal change in lake DOC concentrations.

Light climate and dissolved organic carbon concentration influence species-specific changes in fish zooplanktivory

Dissolved organic carbon (DOC) in lakes reduces light penetration and limits fish production in low nutrient lakes, reportedly via reduced primary and secondary production. Alternatively, DOC and light reductions could influence fish by altering their visual feeding. Previous studies report mixed effects of DOC on feeding rates of zooplanktivorous fish, but most investigators tested effects of a single concentration of DOC against clear-water, turbid, or algal treatments. We used a controlled laboratory study to quantify the effects of a DOC gradient (3–19 mg L−1) on average light climate and the zooplankton feeding rate of 3 common, north temperate fishes. Light availability, which was inversely related to DOC concentration, had a positive and linear effect on zooplankton consumption by juvenile largemouth bass (Micropterus salmoides) and bluegill (Lepomis macrochirus), explaining 22% and 28% of the variation in consumption, respectively. By contrast, zooplankton feeding rates by fathead minnow (Pimephales promelas) were best predicted by a nonlinear, negative influence of light (R2 = 0.13). In bluegill feeding trials we found a general trend for positive selection of larger zooplankton (Cladocera and Chaoboridae); however, the light climate did not influence the selection of prey type. Largemouth bass selected for larger-bodied zooplankton, with weak evidence that selectivity for large Cladocera changed from negative to neutral selection based on electivity values across the light gradient. Our results suggest that the effect of DOC on the light climate of lakes may directly influence fish zooplanktivory and that this influence may vary among fish species.

Dissolved organic carbon modulates mercury concentrations in insect subsidies from streams to terrestrial consumers.

Mercury (Hg) concentrations in aquatic environments have increased globally, exposing consumers of aquatic organisms to high Hg levels. For both aquatic and terrestrial consumers, exposure to Hg depends on their food sources as well as environmental factors influencing Hg bioavailability. The majority of the research on the transfer of methylmercury (MeHg), a toxic and bioaccumulating form of Hg, between aquatic and terrestrial food webs has focused on terrestrial piscivores. However, a gap exists in our understanding of the factors regulating MeHg bioaccumulation by non-piscivorous terrestrial predators, specifically consumers of adult aquatic insects. Because dissolved organic carbon (DOC) binds tightly to MeHg, affecting its transport and availability in aquatic food webs, we hypothesized that DOC affects MeHg transfer from stream food webs to terrestrial predators feeding on emerging adult insects. We tested this hypothesis by collecting data over 2years from 10 low-order streams spanning a broad DOC gradient in the Lake Sunapee watershed in New Hampshire, USA. We found that streamwater MeHg concentration increased linearly with DOC concentration. However, streams with the highest DOC concentrations had emerging stream prey and spiders with lower MeHg concentrations than streams with intermediate DOC concentrations; a pattern that is similar to fish and larval aquatic insects. Furthermore, high MeHg concentrations found in spiders show that MeHg transfer in adult aquatic insects is an overlooked but potentially significant pathway of MeHg bioaccumulation in terrestrial food webs. Our results suggest that although MeHg in water increases with DOC, MeHg concentrations in stream and terrestrial consumers did not consistently increase with increases in streamwater MeHg concentrations. In fact, there was a change from a positive to a negative relationship between aqueous exposure and bioaccumulation at streamwater MeHg concentrations associated with DOC above ~5mg/L. Thus, our study highlights the importance of stream DOC for MeHg dynamics beyond stream boundaries, and shows that factors modulating MeHg bioavailability in aquatic systems can affect the transfer of MeHg to terrestrial predators via aquatic subsidies.

The active microbial diversity drives ecosystem multifunctionality and is physiologically related to carbon availability in Mediterranean semi-arid soils.

Biogeochemical processes and ecosystemic functions are mostly driven by soil microbial communities. However, most methods focus on evaluating the total microbial community and fail to discriminate its active fraction which is linked to soil functionality. Precisely, the activity of the microbial community is strongly limited by the availability of organic carbon (C) in soils under arid and semi-arid climate. Here, we provide a complementary genomic and metaproteomic approach to investigate the relationships between the diversity of the total community, the active diversity and ecosystem functionality across a dissolved organic carbon (DOC) gradient in southeast Spain. DOC correlated with the ecosystem multifunctionality index composed by soil respiration, enzyme activities (urease, alkaline phosphatase and β-glucosidase) and microbial biomass (phospholipid fatty acids, PLFA). This study highlights that the active diversity (determined by metaprotoemics) but not the diversity of the whole microbial community (evaluated by amplicon gene sequencing) is related to the availability of organic C and it is also connected to the ecosystem multifunctionality index. We reveal that DOC shapes the activities of bacterial and fungal populations in Mediterranean semi-arid soils and determines the compartmentalization of functional niches. For instance, Rhizobales thrived at high-DOC sites probably fuelled by metabolism of one-C compounds. Moreover, the analysis of proteins involved in the transport and metabolism of carbohydrates revealed that Ascomycota and Basidiomycota occupied different nutritional niches. The functional mechanisms for niche specialization were not constant across the DOC gradient.

An estimation of diel metabolic rates of eight limnological archetypes from Estonia using high-frequency measurements

We employed a Bayesian model to assess the metabolic state of 8 Estonian lakes representing the 8 lake types according to the European Union Water Framework Directive. We hypothesized that long-term averages of light-related variables would be better predictors of lake metabolism than nutrient-related variables. Model input parameters were in situ high-frequency measurements of dissolved oxygen, temperature, and irradiance. Model simulations were conducted for several (5–12) diel cycles for each lake during the summer season. Accounting for uncertainty, the results from the Bayesian model revealed that 2 lakes were autotrophic for the duration of the experiment, 1 was heterotrophic, and 5 were balanced or had an ambiguous metabolic state. Cross-comparison with a traditional bookkeeping model showed that the majority of lakes were in metabolic balance. A strong coupling between primary production and respiration was observed, with the share of autochthonous primary production respired by consumers increasing with light extinction and nutrient-related variables. Unlike gross primary production, community respiration was strongly related to light extinction, dissolved organic carbon (DOC) and total phosphorus. These findings suggest that a drastic decrease in light-limited primary production along the DOC gradient counter-balanced nutrient supply in the darker lakes and thus blurred the relationship between primary production and nutrients. Thus, contrary to our hypothesis, both light and nutrient-related variables seemed to be good predictors of lake respiration and its coupling to lake primary production.

Habitat, not resource availability, limits consumer production in lake ecosystems

AbstractFood web productivity in lakes can be limited by dissolved organic carbon (DOC), which reduces fish production by limiting the abundance of their zoobenthic prey. We demonstrate that in a set of 10 small, north temperate lakes spanning a wide DOC gradient, these negative effects of high DOC concentrations on zoobenthos production are driven primarily by availability of warm, well‐oxygenated habitat, rather than by light limitation of benthic primary production as previously proposed. There was no significant effect of benthic primary production on zoobenthos production after controlling for oxygen, even though stable isotope analysis indicated that zoobenthos do use this resource. Mean whole‐lake zoobenthos production was lower in high‐DOC lakes with reduced availability of oxygenated habitat, as was fish biomass. These insights improve understanding of lake food webs and inform management in the face of spatial variability and ongoing temporal change in lake DOC concentrations.

A test of the subsidy–stability hypothesis: the effects of terrestrial carbon in aquatic ecosystems

Global change is altering the movement of materials across landscapes in ways that likely have major consequences for the functioning and stability of ecosystems. For example, the export of dissolved organic carbon (DOC) from terrestrial to aquatic ecosystems is increasing globally. This browning phenomenon is expected to alter the stability of recipient aquatic ecosystems, but theory provides contrasting predictions about the form and direction of this response. We created a gradient in terrestrial DOC supply by adding humic substances on weekly basis to 10 experimental ponds (106 L each) over a growing season. The manipulation of terrestrial DOC supply had strong effects on the chemical, physical, and biological properties of the pond ecosystems. Light attenuation linearly increased with terrestrial DOC supply, which created a shading effect that negatively influenced whole‐pond gross primary production and respiration. Despite this, bacterial contributions to basal energy mobilization and respiration increased with terrestrial DOC supply indicating that aquatic food webs were subsidized by terrestrial inputs. After establishing the DOC gradient, we used dynamic linear models to test the subsidy–stability hypothesis by measuring the resilience and sensitivity of each pond to a pulse nutrient perturbation. We found that recovery from the perturbation decreased nonlinearly along a gradient in terrestrial DOC supply. Reciprocal transplant experiments indicated that owing primarily to its light attenuating properties and recalcitrant nature, terrestrial DOC diminished aquatic ecosystem stability by reducing nutrient turnover rates (NTR). Together, our results demonstrate that global‐change‐mediated alterations in the movement of material and energy between habitats can have unpredictable and dramatic impacts on the reliability of ecosystem services.

Taxonomic identity and phytoplankton diet affect fatty acid composition of zooplankton in large lakes with differing dissolved organic carbon concentration

An increase in loading of dissolved organic carbon (DOC) from terrestrial to freshwater ecosystems has recently been documented in several boreal regions. We studied how transfer of important biomolecules, fatty acids, may be connected to DOC in pelagic food webs of large boreal lakes. We collected nine zooplankton taxa during three seasons from six lakes along a gradient of DOC (5–10 mg C L−1). Taxonomic identity explained 62% of the variation in zooplankton fatty acids, while the effects of DOC and season were less pronounced. This suggests that the community composition of zooplankton is important in determining the quality of food available for higher trophic level consumers. Seasonal differences in fatty acid composition were most evident in the taxa that overwinter in an active stage, and especially C18 polyunsaturated fatty acids (PUFA) accumulated toward fall. Small part of the within–taxa variation in zooplankton fatty acids was related to lake DOC concentration. However, the proportion of fatty acids indicative of bacteria or terrestrial vegetation were not linked to DOC. Instead, DOC was connected to PUFA, with zooplankton in high DOC lakes having higher ω3 : ω6 ratios. We propose that this is due to decrease of chlorophytes in phytoplankton community with increasing DOC concentration. Our results with significant among–taxa differences in zooplankton fatty acids imply that terrestrial organic matter may affect the transfer of fatty acids more profoundly if it modifies the zooplankton community composition. However, shifts in the phytoplankton community structure could also cause smaller–scale changes in zooplankton fatty acid composition.

Dissolved organic matter dynamics in the oligo/meso-haline zone of wetland-influenced coastal rivers

Wetlands are key components in the global carbon cycle and export significant amounts of terrestrial carbon to the coastal oceans in the form of dissolved organic carbon (DOC). Conservative behavior along the salinity gradient of DOC and chromophoric dissolved organic matter (CDOM) has often been observed in estuaries from their freshwater end-member (salinity=0) to the ocean (salinity=35). While the oligo/meso-haline (salinity<10) tidal zone of upper estuaries has been suggested to be more complex and locally influenced by geomorphological and hydrological features, the environmental dynamics of dissolved organic matter (DOM) and the environmental drivers controlling its source, transport, and fate have scarcely been evaluated. Here, we investigated the distribution patterns of DOC and CDOM optical properties determined by UV absorbance at 254nm (A254) and excitation–emission matrix (EEM) fluorescence coupled with parallel factor analysis (PARAFAC) along the lower salinity range (salinity<10) of the oligo/meso-haline zone for three distinct wetland-influenced rivers; namely the Bekanbeushi River, a cool-temperate river with estuarine lake in Hokkaido, Japan, the Harney River, a subtropical river with tidally-submerged mangrove fringe in Florida, USA, and the Judan River, a small, acidic, tropical rainforest river in Borneo, Malaysia. For the first two rivers, a clear decoupling between DOC and A254 was observed, while these parameters showed similar conservative behavior for the third. Three distinct EEM-PARAFAC models established for each of the rivers provided similar spectroscopic characteristics except for some unique fluorescence features observed for the Judan River. The distribution patterns of PARAFAC components suggested that the inputs from plankton and/or submerged aquatic vegetation can be important in the Bekanbeushi River. Further, DOM photo-products formed in the estuarine lake were also found to be transported upstream. In the Harney River, whereas upriver-derived terrestrial humic-like components were mostly distributed conservatively, some of these components were also derived from mangrove inputs in the oligo/meso-haline zone. Interestingly, fluorescence intensities of some terrestrial humic-like components increased with salinity for the Judan River possibly due to changes in the dissociation state of acidic functional groups and/or increase in the fluorescence quantum yield along the salinity gradient. The protein-like and microbial humic-like components were distributed differently between three wetland rivers, implying that interplay between loss to microbial degradation and inputs from diverse sources are different for the three wetland-influenced rivers. The results presented here indicate that upper estuarine oligo/meso-haline regions of coastal wetland rivers are highly dynamic with regard to the biogeochemical behavior of DOM.

Stable isotope variability of meso‐zooplankton along a gradient of dissolved organic carbon

Summary1. The δ13C and δ15N signatures of zooplankton vary with dissolved organic carbon (DOC), but inconsistent and limited taxonomic resolution of previous studies have masked differences that may exist among orders, genera or species and are attributable to dietary and/or habitat differences. Here we investigate differences among the isotopic signatures of five zooplankton taxa (Daphnia, Holopedium, large Calanoida, small Calanoida and Cyclopoida) in Precambrian shield lakes with a sixfold range of DOC concentration.2. δ13C signatures of Daphnia, small calanoids and large calanoids became more depleted with increasing lake DOC, whereas Holopedium and cyclopoid δ13C became enriched with increasing DOC concentration.3. The variability of δ13C and δ15N isotopic signatures among zooplankton groups was reduced in high‐DOC, compared to low‐DOC lakes, especially for δ13C. Differences in δ13C and POM‐corrected δ15N accounted for up to 33.7% and 19.5% of the variance, respectively, among lakes of varying DOC concentration.4. The narrow range of signatures found in higher DOC lakes suggests that different taxa have similar food sources and/or habitats. In contrast, the wide range of signatures in low‐DOC lakes suggests that different taxa are exploiting different food sources and/or habitats. Together with the variable trends in zooplankton isotopic signatures along our DOC gradient, these results suggest that food web dynamics within the zooplankton community of temperate lakes will change as climate and lake DOC concentrations change.