Macarena S. Valiñas

Habitat use and feeding habits of juvenile fishes in an infrequently flooded Atlantic saltmarsh

In saltmarshes, marsh creeks provide an important corridor between the marsh and the subtidal habitat for fishes. We compare fish and prey in a Spartina densiflora marsh creek with a tidal flat in the SW Atlantic (Argentina) to evaluate the hypotheses that: (1) benthic prey abundance is higher in the marsh creek, and therefore the abundance of benthivorous fishes and predation pressure on benthos is higher in this area; and (2) marsh creeks act as refuge areas for fishes. Fish abundance and benthic prey availability were sampled over four seasons, and dietary composition of Odontesthes argentinensis and Micropogonias furnieri was assessed. Brevoortia aurea was more abundant in the marsh creek, Micropogonias furnieri showed the opposite pattern, and Odontesthes argentinensis and Ramnogaster arcuata did not dominate either habitat. As expected, smaller fishes were more abundant in the marsh creek. Food abundance was higher in the marsh creek but only M. furnieri consumed more prey in this area, while O. argentinensis consumed more in the tidal flat. Differences in prey accessibility and sediment features between areas could explain these results. This work highlights the importance of marsh creeks as refuge and/or feeding grounds for fishes in infrequently flooded saltmarshes.

First record of the sea anemone Diadumene lineata (Verrill 1871) associated to Spartina alterniflora roots and stems, in marshes at the Bahia Blanca estuary, Argentina

We report the occurrence of the orange-striped green anemone Diadumene lineata (Verrill 1871) (=Haliplanella lineata) in salt marshes at the Bahía Blanca Estuary for the first time in August 2005. We also found this species attached to roots and stems of Spartina alterniflora, an association that has never been registered before. After their determination, sampling was performed during a year to evaluate seasonal abundance of this sea anemone. Results showed that D. lineata was present through the whole year, indicating the existence of a stable population. All individuals sampled were found attached to roots or stems of S. alterniflora, with the higher abundances detected in summer. Further studies are necessary to precise the potential effects of this exotic sea anemone on salt marsh communities.

Solar UVR sensitivity of phyto- and bacterioplankton communities from Patagonian coastal waters under increased nutrients and acidification

&NA; The effects of ultraviolet radiation (UVR) under future expected conditions of acidification and increase in nutrient inputs were studied on a post‐bloom phytoplankton and bacterioplankton community of Patagonian coastal waters. We performed an experiment using microcosms where two environmental conditions were mimicked using a cluster approach: present (ambient nutrients and pH) and future (increased nutrients and acidification), and acclimating the samples for five days to two radiation treatments (full solar radiation [+UVR] and exclusion of UVR [‐UVR]). We evaluated the short‐term (hours) sensitivity of the community to solar UVR through chlorophyll a fluorescence parameters (e.g. the effective photochemical quantum yield of PSII [&PHgr;PSII]) at the beginning, at the mid‐point and at the end of the acclimation period. Primary production and heterotrophic bacterial production (HBP) were determined, and biological weighting functions were calculated, at the beginning and at the end of the acclimation period. Mid‐term effects (days) were evaluated as changes in taxonomic composition, growth rates and size structure of the community. Although the UVR‐induced inhibition on &PHgr;PSII decreased in both clusters, samples remained sensitive to UVR after the 5 days of acclimation. Also, under the future conditions, there was, in general, an increase in the phytoplankton carbon incorporation rates along the experiment as compared to the present conditions. Bacterioplankton sensitivity to UVR changed along the experiment from inhibition to enhancement of HBP, and future environmental conditions stimulated bacterial growth, probably due to indirect effects caused by phytoplankton. Those changes in the microbial loop functioning and structure under future global change conditions might have important consequences for the carbon pump and thus for the carbon sequestration and trophodynamics of Patagonian coastal waters.

Dual role of DOM in a scenario of global change on photosynthesis and structure of coastal phytoplankton from the South Atlantic Ocean

We evaluated the dual role of DOM (i.e., as a source of inorganic nutrients and as an absorber of solar radiation) on a phytoplankton community of the western South Atlantic Ocean. Using a combination of microcosms and a cluster approach, we simulated the future conditions of some variables that are highly influenced by global change in the region. We increased nutrients (i.e., anthropogenic input) and dissolved organic matter (DOM), and we decreased the pH, to assess their combined impact on growth rates (μ), species composition/abundance and size structure, and photosynthesis (considering in this later also the effects of light quality i.e., with and without ultraviolet radiation). We simulated two Future conditions (Fut) where nutrients and pH were similarly manipulated, but in one the physical role of DOM (Futout) was assessed whereas in the other (Futin) the physico-chemical role was evaluated; these conditions were compared with a control (Present condition, Pres). The μ significantly increased in both Fut conditions as compared to the Pres, probably due to the nutrient addition and acidification in the former. The highest μ were observed in the Futout, due to the growth of nanoplanktonic flagellates and diatoms. Cells in the Futin were photosynthetically less efficient as compared to those of the Futout and Pres, but these physiological differences, also between samples with or without solar UVR observed at the beginning of the experiment, decreased with time hinting for an acclimation process. The knowledge of the relative importance of both roles of DOM is especially important for coastal areas that are expected to receive higher inputs and will be more acidified in the future.

Global change effects on plankton community structure and trophic interactions in a Patagonian freshwater eutrophic system

The short- and mid-term effects of a simulated global change scenario (i.e., Future) of increased nutrients, acidification, and solar radiation, in the presence or absence of grazers, were evaluated on a freshwater plankton community of Patagonia, Argentina. We used a cluster experimental design with microcosms incubated outdoors simulating the in situ (i.e., Present) and the Future conditions. Short-term changes in net productivity and respiration, together with mid-term changes in the community (abundance, biomass, and phytoplankton cell size) were measured. Phytoplankton had lower net productivity and higher respiration and zooplankton had, in general, higher respiration under the Future than that under the Present condition when organisms were exposed to UVR. The mid-term impacts of the Future condition were neither significant on zooplankton abundances, nor in phytoplankton abundances, biomass, and cell size. Nevertheless, the zooplankton–phytoplankton interaction strength was greater under the Future condition. Zooplankton exerted a strong top-down pressure, regardless of the experimental scenarios, grazing preferentially on small phytoplankton cells, thus decreasing their abundances and biomass. Overall, there were significant short-term impact of our Future global change scenario; however, its effects on mid-term time scales were not significant, and indeed, the zooplankton top-down pressure was the main driver that shaped the phytoplankton community.

Effect of “Whitemouth Croaker” (Micropogonias furnieri, Pisces) on the Stability of the Sediment of Salt Marshes—an Issue To Be Resolved

Among the predators, fish are prevalent in intertidal soft-bottom zones, and many create substantial interruptions in the sediment surface through their feeding, thus affecting the movement of fluids in the sediment-water interface and therefore the rates of deposition and local erosion. This study was designed to determine whether or not Micropogonias furnieri—an ecologically significant benthophagic southwestern Atlantic Ocean predator—modified erosion and/or sedimentation processes in salt marshes. The results indicated that this species exhibited a preference for areas without vegetation cover at the time of feeding since a greater abundance of pits was found in those environments. Moreover, the volume analysis of the pits in the two areas indicated that the size of the fish that had foraged in the sediment was significantly larger in the nonvegetated areas. The results of the M. furnieri-exclusion experiment indicated that the presence of this sciaenid neither resulted in a decrease in benthic organisms in the nonvegetated areas nor affected the vertical distribution of the infauna. When M. furnieri was excluded, the sediment exhibited higher critical-shearing and frictional-velocity values than in areas where M. furnieri had access and therefore was less likely to be eroded. The data from these experiments enabled us to conclude that the foraging action of M. furnieri modified the stability of the sediment as a result of the predatory pressure that the fish exerted on the organisms inhabiting the salt marshes, thus resulting in the generation of elliptical depressions. That modification of the sediment stability was evidenced in two principal ways: (i) a negative effect on the microphytobenthic organisms that decreased the concentration of extracellular polymeric substances in the sediment and (ii) an increased roughness of the bottom and increased percentage of sand in the particle composition of the sediment, where the fish had foraged.