Salinity is a well-recognised factor that shapes population dynamics and community structure through direct and indirect effects. The responses of Moina micrura (Cladocera) to slight salinity increases (up to 6.0) were evaluated through laboratory manipulative experiments and field observations at Cabiúnas lagoon (Rio de Janeiro, Brazil). From the field data, a positive relationship between M. micrura density and salinity was observed, whereas the opposite was found for other co-occurring microcrustaceans. Life-table observations from laboratory experiments showed better performance of individuals exposed to control conditions (no salt addition) and 1.0 and 2.0 of salinity. The difference between field and laboratory results may be explained considering possible direct and indirect effects of salinity on M. micrura populations. Salinity may have direct negative physiological effects in individuals. However, under natural conditions, M. micrura might benefit from reduced invertebrate predation and interspecific competition. This balance between negative and positive effects of salinity is probably more positive under natural conditions, explaining the success of M. micrura in Cabiúnas lagoon under mild salinity conditions. Although salinity negatively affects many species living in freshwater systems, slight increases may be beneficial for some species under natural conditions.
Element cycling in aquatic systems is driven chiefly by planktonic processes, and the structure of the planktonic food web determines the efficiency of carbon transfer through trophic levels. However, few studies have comprehensively evaluated all planktonic food-web components in tropical regions. The aim of this study was to unravel the top-down controls (metazooplankton community structure), bottom-up controls (resource availability), and hydrologic (water residence time) and physical (temperature) variables that affect different components of the microbial food web (MFW) carbon stock in tropical reservoirs, through structural equation models (SEM). We conducted a field study in four deep Brazilian reservoirs (Balbina, Tucuruí, Três Marias, and Funil) with different trophic states (oligo-, meso-, and eutrophic). We found evidence of a high contribution of the MFW (up to 50% of total planktonic carbon), especially in the less-eutrophic reservoirs (Balbina and Tucuruí). Bottom-up and top-down effects assessed through SEM indicated negative interactions between soluble reactive phosphorus and phototrophic picoplankton (PPP), dissolved inorganic nitrogen, and heterotrophic nanoflagellates (HNF). Copepods positively affected ciliates, and cladocerans positively affected heterotrophic bacteria (HB) and PPP. Higher copepod/cladoceran ratios and an indirect positive effect of copepods on HB might strengthen HB-HNF coupling. We also found low values for the degree of uncoupling (D) and a low HNF/HB ratio compared with literature data (mostly from temperate regions). This study demonstrates the importance of evaluating the whole size spectrum (including microbial compartments) of the different planktonic compartments, in order to capture the complex carbon dynamics of tropical aquatic ecosystems.
Aim: Artificial sandbar openings are a common management practice in coastal lagoons but they can be a threat when negative effects to the quality of water and to the aquatic biota are observed. The current study compared sandbar opening effects in two coastal lagoons located close to each other, but differing on trophic status and on sandbar openings' background. Methods: Limnological variables and zooplankton community were recorded monthly during one year before and one year after sandbar openings that occurred in the same month for both lagoons, giving 24 samples. We compared the effects of sandbar opening on response variables, according to the two types of system. Results: The sandbar openings determined changes in some limnological features -depth and salinity -but such effects were different in the two types of system. The zooplankton structure displayed dramatic changes in the eutrophic and commonly opened lagoon. The occurrence and abundance of some species were closely related to changes in limnological variables. Conclusions: Our data indicated that zooplankton communities are more resistant to sandbar openings in coastal lagoons historically less disturbed. The direction and magnitude of changes promoted by sandbar openings might be specific to each lagoon, due to different backgrounds of disturbances that, in the long term, modify the water quality and the structure of zooplankton communities, and consequently, their resistance and resilience.
The present study investigated the intra-ecosystem variability of nutrient enrichment (nitrogen -N- and phosphorus -P-), density of faecal coliforms (i.e., as a proxy of sanitary conditions), and ecological responses of the zooplankton community structure in a tropical urban coastal lagoon, following eutrophication, changes in rainfall, and episodic sandbar openings. Surface waters were monthly taken over 14 years (1992-2005) within the long-term monitoring program ECOlagoas from two sampling stations at the Imboassica lagoon (Northern of the Rio de Janeiro State, Brazil): near a domestic wastewater channel (SEWAGE station), and at the limit to the sea (SANDBAR station). Changes in faecal coliforms and nutrient concentration reduced water quality due to untreated sewage inputs, which was intensified by artificial sandbar openings mainly under low rainfall, such as observed in February 2001. Indeed, a greater depth and subsequent volume of water of the Imboassica lagoon during the acidification period (i.e., attributed to the increased rainfall, and lower frequency of sandbar openings) contributed to dilute the faecal coliform only in the SANDBAR station. In turn, the zooplankton community structure showed that episodic sandbar openings promoted the loss of their functional features derived from a reduction and replacement of species over time, even in the period of lower frequency of sandbar opening and dilution of nutrient concentrations. Our findings indicate that sandbar openings are not the best management practice when facing coastal eutrophication, once it may cause harmful effects on sanitary conditions and ecological community related to zooplankton, mainly in periods of decline in rainfall.
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