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Bottom trawling is one of the most harmful and widespread activities affecting benthic habitats and fauna. In this study, we analyzed the impact of shrimp trawling fisheries on the densities, assemblage structure and vertical stratification of the benthic macrofauna in shallow mudflats of the Patos Lagoon Estuary (Brazil). Experimental trawls of different intensities were performed during three shrimp fishing seasons (2015–2017), comparing macrofauna among before and after trawling, and a control zone. The changes in macrofaunal assemblages were more consistently related to natural variability than to trawling impact, being mostly influenced by sediment structure and salinity variation. The trawling impact was mainly detected in the area with higher percentages of fine sediments, with different macrofaunal responses in each month and stratum. Some non-significant decreases on total densities after trawling were observed, mainly on the superficial stratum, and signs of burial activities. The response to trawling disturbance of each species was different and it highly depended on their natural variability. Only five species showed significant variation to the trawling treatment ( Erodona mactroides , Heleobia australis, H. charruana, Heteromastus similis and Laeonereis acuta ), with temporally different responses. Decreasing densities were more related to the high impact treatment. In some seasons, decreases on the abundance of Monokalliapseudes schubarti after high impact trawls were up to 60% of the seasonal mean densities. Trawling fisheries may reduce macrobenthic densities but not their vertical stratification. The impact of trawling on key species may compromise the ecosystem function, as benthic macrofauna provide food sources for many aquatic resources. This study highlights trawling impacts in an estuarine nursery area, which should be fully considered since they may be reflected in the trophic webs. [ABSTRACT FROM AUTHOR]

In this study it was hypothesised that increasing water retention time promotes phytoplankton blooms in the shallow microtidal Patos Lagoon estuary (PLE). This hypothesis was tested using salinity variation as a proxy of water retention time and chlorophyll a for phytoplankton biomass. Submersible sensors fixed at 5 m depth near the mouth of PLE continuously measured water temperature, salinity and pigments fluorescence (calibrated to chlorophyll a ) between March 2010 and 12th of December 2011, with some gaps. Salinity variations were used to separate alternating patterns of outflow of lagoon water (salinity <8; 46% of the time) and inflow of marine water (salinity >24; 35% of the time). The two transition phases represented a rapid change from lagoon water outflow to marine water inflow and a more gradually declining salinity between the dominating inflow and outflow conditions. During the latter of these, a significant chlorophyll a increase relative to that expected from a linear mixing relationship was observed at intermediate salinities (10–20). The increase in chlorophyll a was positively related to the duration of the prior coastal water inflow in the PLE. Moreover, chlorophyll a increase was significantly higher during austral spring-summer than autumn-winter, probably due to higher light and nutrient availability in the former. Moreover, the retention time process operating on time scales of days influences the long-term phytoplankton variability in this ecosystem. Comparing these results with monthly data from a nearby long-term water quality monitoring station (1993–2011) support the hypothesis that chlorophyll a accumulations occur after marine inflow events, whereas phytoplankton does not accumulate during high water outflow, when the water residence time is short. These results suggest that changing hydrological pattern is the most important mechanism underlying phytoplankton blooms in the PLE. [ABSTRACT FROM AUTHOR]

Seasonal and interannual changes (1993–2012) of water temperature and transparency, river discharge, salinity, water quality properties, chlorophyll a (chl- a ) and the carbon biomass of the main taxonomical phytoplankton groups were evaluated at a shallow station (∼2 m) in the subtropical Patos Lagoon Estuary (PLE), Brazil. Large variations in salinity (0–35), due to a complex balance between Patos Lagoon outflow and oceanic inflows, affected significantly other water quality variables and phytoplankton dynamics, masking seasonal and interannual variability. Therefore, salinity effect was filtered out by means of a Generalized Additive Model (GAM). River discharge and salinity had a significant negative relation, with river discharge being highest and salinity lowest during July to October. Diatoms comprised the dominant phytoplankton group, contributing substantially to the seasonal cycle of chl- a showing higher values in austral spring/summer (September to April) and lowest in autumn/winter (May to August). PLE is a nutrient-rich estuary and the phytoplankton seasonal cycle was largely driven by light availability, with few exceptions in winter. Most variables exhibited large interannual variability. When varying salinity effect was accounted for, chl- a concentration and diatom biomass showed less irregularity over time, and significant increasing trends emerged for dinoflagellates and cyanobacteria. Long-term changes in phytoplankton and water quality were strongly related to variations in salinity, largely driven by freshwater discharge influenced by climatic variability, most pronounced for ENSO events. However, the significant increasing trend of the N:P ratio indicates that important environmental changes related to anthropogenic effects are undergoing, in addition to the hydrology in the PLE. [ABSTRACT FROM AUTHOR]

Abstract: With a surface area of over 10,000 km2, Patos Lagoon is Brazil''s largest (choked) coastal lagoon. Directly or indirectly, Patos is associated with two other coastal lagoons, Mirim and Mangueira. Patos Lagoon reaches maximum water level height during the austral winter, in coherence with the rainy season. The longest (1961–2011) available rainfall (recorded at nearby Porto Alegre, Southern Brazil) and river discharge time series (1940–2011, in the tributary Jacuí River, at Rio Pardo) shows an overall increasing trend through the application of Mann–Kendall analysis. If, however, the series are split in two segments, negative trends become evident for the 1990–2011 period; in both cases earlier data showed a positive trend until ∼1989. Coherent with water inflow, the lagoon''s water height level shows a seasonal Kendall''s τ coefficient that is consistently negative for the month of April (statistically significant p). Rainfall over Patos'' drainage basin is actively teleconnected with ENSO occurrences in the equatorial Pacific. This can be verified in Porto Alegre''s rainfall as well as in the Jacuí River discharge time series and in the lagoon''s water level variability; a distinctly negative Southern Oscillation Index (SOI) usually results in excess precipitation, high riverine discharge and, accordingly, above normal lagoon water level. The use of a method for harmonic analysis (Continuous Wavelet Transform or CWT) allows detecting decadal and inter-annual periodicities in the rainfall and discharge frequencies and in the lagoon''s water level time series. Also, a change in the frequency pattern appears to have occurred in the late 1990s (i.e., simultaneous with the 1997 ENSO event?) and suggests that it may be connected with the trend change which resulted in the current negative slope observed in deseasonalized hydrological data and a fainter ENSO signal for the region. [Copyright &y& Elsevier]

High rainfall associated with El Nin˜o events significantly increases runoff and stream discharge in southern Brazil. High freshwater discharge changes salinity, temperature, and water circulation patterns that can affect the fish estuarine assemblage. Using long-term data obtained from standardized surveys, we analyzed fish assemblage structure and dynamics in shallow waters of the Patos Lagoon estuary in southern Brazil before, during, and after the 1997–1998 El Nin˜o event. Overall, the relative abundance of all the fish groups in the estuary was about five times lower during the El Nin˜o than before and after. Freshwater vagrants were the only group with greater abundance during El Nin˜o. Fish species richness was higher in the estuary during the El Nin˜o event, when many freshwater species expanded their ranges into the Patos Lagoon estuary, than before or after the El Nin˜o. El Nin˜o-induced assemblage changes were not highly persistent, and the estuarine fish assemblage returned to its pre-El Nin˜o state within 18 months after the El Nin˜o period. Densities of many marine and estuarine fishes increased to pre-El Nin˜o levels within 3–6 months of the end of the El Nin˜o period. We suggested that the rapid recovery of fish estuarine populations after the 1997–1998 El Nin˜o may have been caused by one or some combination of: (a) enhanced productivity stimulated by nutrients contained in newly deposited alluvial sediments, and (b) enhanced larvae transport in the large saltwater intrusion that followed the El Nin˜o event. Clearly, fish population dynamics and assemblage structure of the Patos Lagoon estuary can neither be interpreted nor predicted on a long-term basis without explicit consideration of El Nin˜o Southern Oscillation patterns. [Copyright &y& Elsevier]

Abundance and respiration of free and attached microorganisms were monitored during the decomposition of the seagrass Scirpus maritimus leaves in laboratory microcosms for 30 days. There was a clear succession between bacteria and heterotrophic flagellates during the course of the study. The beginning of the study (1–4 days) was characterized by higher rates of bacterial respiration, compared to the later periods. Free microorganisms were responsible for more than half of the respiration (65%) within the microcosms, suggesting that they were responsible for the mineralization of the bulk of the macrophyte detritus following its dissolution. On the other hand, estimates of activity on a per cell basis revealed that individual attached bacteria had much higher (3- to 4-fold) respiration rates than free bacteria, suggesting attached bacterial activity may play a key role in the breakdown and dissolution of particulate detritus in estuarine waters. The findings suggest different but coupled roles for attached and free bacteria in nature. [Copyright &y& Elsevier]

Trawling fisheries cause in their path uncounted impacts, including direct and indirect effects. This study aimed to evaluate the impact of artisanal trawls on the food availability and its possible impact on the diet of two commercial crustacean species, the pink-shrimp Penaeus paulensis and the blue crab Callinectes sapidus. Two mesocosms were constructed (control and simulated fishing), where sediment samples were collected to analyze the zoobenthos, sediment grain sizes and the percentage of organic matter. Sediment samples were separated into two strata (0–5 and 5–10 cm depth) to evaluate the vertical structure of the fauna, the availability of prey and possible shifts in these patterns after trawling. The Prey-Specific Index of Relative Importance was used in stomach contents analyses. Significant decreases in the percentage of organic matter were observed before and after trawling. Perturbations caused by trawling activities resulted in alterations on the superficial layer, with significant changes in zoobenthos total abundance induced by resuspension and settlement of organisms in other places. Changes in the abundance and richness of zoobenthic organisms caused by trawling were reflected in the diet of pink shrimp and blue crab. A decrease in the main ingested organisms and an increase in the ingestion of more refractory and less energetic items (mainly plants) occurred. Overall, there was a significant effect of trawling on benthic organisms, in particular cases reflected in the predators stomach content, further confirming the impact of trawling on estuarine trophic webs. • Destructuration of the sediment strata occurred after the otter disturbance. • Alterations on benthic habitat affected macrobenthos and megabenthos. • Trawling caused shifts in the diet of megabenthic organisms. • The ingestion of the main preys was changed for less energetic items. [ABSTRACT FROM AUTHOR]