Salinity increases zooplanktonic grazing rates through metabolism adjustments

Background/Question/Methods

Salinity is considered a key factor shaping zooplankton community structure in coastal aquatic ecosystems, where even slight salinity increases may negatively affect estuarine organisms. Changes on salinity occur in response to variations in the sea level, freshwater recharge (precipitation or hydrological flows) and evaporation. Effects of salinity variation on zooplankton communities have been widely explored; however few studies have analyzed its consequences to ecosystems functions. Furthermore, these studies were not able to separate the effects of changes on community composition from those related to metabolic adjustments. Here we performed a salinity increasing experiment, in order to analyze changes on the community structure and the effects on zooplanktonic grazing rates through community composition changes and metabolism regulation. Zooplanktonic species were isolated from Cabiúnas lagoon, located in the Restinga de Jurubatiba National Park, (RJ, Brazil) and regrown under three salinity levels (0.1- no salt addition, 2.0 and 6.0 of salinity) for five days. After this period, grazing rates were measured using zooplanktonic communities that survived to each salinity level. Measurement was made in natural conditions (salinity of 0.1) and under salinity influence (salinity of 2.0 and 6.0) to isolate community composition from metabolic adjustments effects. Grazing rates were obtained by estimating the consumption of three phytoplanktonic species (Ankistrodesmus gracilis, Scenedesmus bijugatus, and Pseudokirchneriella subcapitata) by the zooplankton community during one hour in the dark.

Results/Conclusions

We found 16 species distributed into three major zooplanktonic groups: Copepoda, Cladocera, and Rotifera. Salinity decreased species diversity, mainly at 6.0, in which Copepods groups (salt-tolerant) replaced Cladocerans ones (salt-intolerant) in dominance. Additionally, salinity was able to explain 77% of all variation in treatments (p<0.001, R²=0.77, PERMANOVA). We observed an increase of grazing rates of communities at 6.0 of salinity when compared to communities at 2.0, at natural conditions, and even at natural conditions but previously exposed to 6.0 (p<0.05, ANOVA  plus Tukey test, and p<0.05, Paired T-test). We hypothesized that individuals exposed to higher salinities may increase food consumption due to the necessity of supplementary energy for osmoregulation. Even with more efficient filter-feeders (Cladocerans) loss, an increase on grazing rates was observed, corroborating the metabolic adjustments hypothesis. We conclude that both community composition changes and metabolic adjustments caused by salinity increasing affect zooplanktonic grazing rates, but the effects of the latter seem to be stronger than the former.