The fate of nitrogen in the Zarin-Gol River receiving trout farm effluent.

This study investigated the Zarrin-Gol River ecosystem in Iran to trace organic matter in the food web and evaluate the impact of aquaculture farm effluent using stable isotopes of nitrogen (δ15N) and carbon (δ13C). Using a previously-developed model (Islam 2005), we estimated that a trout farm in the vicinity released 1.4 tons of nitrogen into the river. This was comparable to an estimated total nutrient load of 2.1 tons of nitrogen for the six-month fish-rearing period based on a web-based constituent load estimator (LOADEST). A model estimate of river nitrogen concentration at the time of minimum river discharge (100 L/s) was 2.74 mg/L. Despite relatively high nitrogen loading from the farm, isotope data showed typical food web structure. Several biological groups had elevated δ13C or δ15N values, but there was limited evidence for the entry of organic matter from the trout farm into the food web, with sites above and below trout farms having inconsistent patterns in 15N enrichment. By coupling nitrogen load modeling with stable isotope analysis we showed that stable isotopes might not be effective tracers of organic matter into food webs, depending on surrounding land use and other point sources of nutrients. The Zarrin-Gol River ecosystem, like other basins with high human population density, remains vulnerable to eutrophication in part due to trout farm effluent.

Assessing nitrogen dynamics model and the role of artificial lagoon in effluent loading of shrimp farms in Gomishan wetland, southern Caspian Sea.

Shrimp farming increases the nutrients, especially nitrogen in the water resources reducing water quality. This study was conducted to evaluate the nitrogen dynamics in white shrimp (Litopenaeus vannamei) farms and the role of artificial lagoon (24 ha) in reducing nitrogen levels in Gomishan coastal wetland, the eastern Caspian Sea. The results indicated that at the end of the 4-month breeding period, the amounts of nitrogen and phosphorus introduced into Gomishan wetland were calculated as to 220.157 and 39 tons, respectively in a breeding area covering 830 hectares. Nitrogen values (based on nitrate) calculated based on the relationship between the basin and the discharge of the outlet channel of the site at the time of complete emptying of the farms, were calculated to be approximately 121.8 tons per breeding time that it had an important role in eliminating about 45% of nutrient pollution and reducing the concentration of dissolved nitrogen. Moreover, nitrogen isotopic trace was observed in shrimp samples, in similar levels in the samples of both shrimp pond and lagoon, which emphasizes the role of feeding from natural food, especially benthic fauna. Overall, due to the decline of Caspian Sea water level, Gomishan coastal wetland is drying, and the output of shrimp farms is currenly the only source supplying water for the wetland. Hence, appropriate management strategies could minimize the amounts of nutrients into the natural water whilst aiding wetland's survaival.

Spatial and temporal variations in stable isotope values (δ13C and δ15N) of the primary and secondary consumers along the southern coastline of the Caspian Sea.

Spatial and temporal variations in stable isotope values of the coastal communities in the Caspian Sea remain poorly understood. We followed carbon stable isotope ratios (δ13C) and nitrogen stable isotope ratios (δ15N) of primary (gammarids) and secondary (common carp) consumers at four sites with different organic matter loadings along the southern coastline of the Caspian Sea from May to November 2019. Consumers from the site receiving higher sewage effluent and aquaculture wastes were more enriched in δ15N whereas those from the site receiving lower terrestrial-derived organic matter had enriched δ13C values. Trophic position of consumers varied among sites as a function of organic matter inputs but was independent of month. The observed temporal and spatial variations in the stable isotope values and C:N ratios of the primary and secondary consumers with regards to organic matter loadings, suggesting a possible effect of anthropogenic disturbance on the Caspian Sea ecosystem function.

Non-indigenous species refined national baseline inventories: A synthesis in the context of the European Union's Marine Strategy Framework Directive.

Refined baseline inventories of non-indigenous species (NIS) are set per European Union Member State (MS), in the context of the Marine Strategy Framework Directive (MSFD). The inventories are based on the initial assessment of the MSFD (2012) and the updated data of the European Alien Species Information Network, in collaboration with NIS experts appointed by the MSs. The analysis revealed that a large number of NIS was not reported from the initial assessments. Moreover, several NIS initially listed are currently considered as native in Europe or were proven to be historical misreportings. The refined baseline inventories constitute a milestone for the MSFD Descriptor 2 implementation, providing an improved basis for reporting new NIS introductions, facilitating the MSFD D2 assessment. In addition, the inventories can help MSs in the establishment of monitoring systems of targeted NIS, and foster cooperation on monitoring of NIS across or within shared marine subregions.

A global database of nitrogen and phosphorus excretion rates of aquatic animals.

Animals can be important in modulating ecosystem-level nutrient cycling, although their importance varies greatly among species and ecosystems. Nutrient cycling rates of individual animals represent valuable data for testing the predictions of important frameworks such as the Metabolic Theory of Ecology (MTE) and ecological stoichiometry (ES). They also represent an important set of functional traits that may reflect both environmental and phylogenetic influences. Over the past two decades, studies of animal-mediated nutrient cycling have increased dramatically, especially in aquatic ecosystems. Here we present a global compilation of aquatic animal nutrient excretion rates. The dataset includes 10,534 observations from freshwater and marine animals of N and/or P excretion rates. These observations represent 491 species, including most aquatic phyla. Coverage varies greatly among phyla and other taxonomic levels. The dataset includes information on animal body size, ambient temperature, taxonomic affiliations, and animal body N:P. This data set was used to test predictions of MTE and ES, as described in Vanni and McIntyre (2016; Ecology DOI: 10.1002/ecy.1582).

Predator diversity effects in an exotic freshwater food web.

Cascading trophic interactions are often defined as the indirect effects of a predator on primary producers through the effect of the predator on herbivores. These effects can be both direct through removal of herbivores [density-mediated indirect interactions (DMIIs)] or indirect through changes in the behavior of the herbivores [trait-mediated indirect interactions (TMIIs)]. How the relative importance of these two indirect interactions varies with predator diversity remains poorly understood. We tested the effect of predator diversity on both TMIIs and DMIIs on phytoplankton using two competitive invasive dreissenid mussel species (zebra mussel and quagga mussel) as the herbivores and combinations of one, two or all three species of the predators pumpkinseed sunfish, round goby, and rusty crayfish. Predators had either direct access to mussels and induced both TMII and DMII, or no direct access and induced only TMII through the presence of risk cues. In both sets of treatments, the predators induced a trophic cascade which resulted in more phytoplankton remaining with predators present than with only mussels present. The trophic cascade was weaker in three-predator and two-predator treatments than in one-predator treatments when predators had direct access to dreissenids (DMIIs and TMIIs). Crayfish had higher cascading effects on phytoplankton than both pumpkinseed and round goby. Increased predator diversity decreased the strength of DMIIs but had no effect on the strength of TMIIs. The strength of TMIIs was higher with zebra than quagga mussels. Our study suggests that inter-specific interference among predators in multi-species treatments weakens the consumptive cascading effects of predation on lower trophic levels whereas the importance of predator diversity on trait mediated effects depends on predator identity.

Stoichiometric constraints do not limit successful invaders: zebra mussels in Swedish lakes.

BACKGROUND: Elemental imbalances of carbon (C): nitrogen (N): phosphorus (P) ratios in food resources can constrain the growth of grazers owning to tight coupling between growth rate, RNA allocation and biomass P content in animals. Testing for stoichiometric constraints among invasive species is a novel challenge in invasion ecology to unravel how a successful invader tackles ecological barriers in novel ecosystems. METHODOLOGY/PRINCIPAL FINDINGS: We examined the C:P and N:P ratios and the condition factor of a successful invader in lakes, the zebra mussel (Dreissena polymorpha), collected from two Swedish lakes. Concurrently, we analyzed the elemental composition of the food (seston) and tissue of the mussels in which nutrient composition of food and mussels varied over time. Zebra mussel condition factor was weakly related to the their own tissue N:P and C:P ratios, although the relation with the later ratio was not significant. Smaller mussels had relatively lower tissue N:P ratio and higher condition factor. There was no difference in C:P and N:P ratios between seston and mussels' tissues. Our results indicated that the variation in nutrient stoichiometry of zebra mussels can be explained by food quality and quantity. CONCLUSIONS/SIGNIFICANCE: Our study suggests that fitness of invasive zebra mussels is not constrained by nutrient stoichiometry which is likely to be important for their proliferation in novel ecosystems. The lack of imbalance in C:P and N:P ratios between seston and mussels along with high tissue C:P ratio of the mussel allow them to tolerate potential P limitation and maintain high growth rate. Moreover, zebra mussels are able to change their tissue C:P and N:P ratios in response to the variation in elemental composition of their food. This can also help them to bypass potential nutrient stoichiometric constraints. Our finding is an important step towards understanding the mechanisms contributing to the success of exotic species from stoichiometric principles.