Afonso L. Prestes

Differences in the structure and functioning of two communities: Frondose and turf-forming macroalgal dominated habitats

In many coastal regions, vegetated habitats (e.g. kelps forests, seagrass beds) play a key role in the structure and functioning of shallow subtidal reef ecosystems, by modifying local environmental conditions and by providing food and habitat for a wide range of organisms. In some regions of the world, however, such idiosyncratic ecosystems are largely absent and are often replaced by less notable ecosystem formers. In the present study, we empirically compared the structure and functioning of two distinct shallow-water habitats present in the Azores: one dominated by smaller frondose brown macroalgae (Dictyotaceae and Halopteris) and one dominated by low-lying turfs. Two replicated areas of each habitat were sampled at two different times of the year, to assess spatial and temporal consistency of results. Habitats dominated by small fronds were significantly (ca. 3 times) more productive (when standardized per algal mass) compared to the turf-dominated habitats, and supported a distinct assemblage (both in terms of composition and abundance) of associated macrofauna. Unlike other well-known and studied vegetated habitats (i.e. kelp forests), however, no effects of habitat were found on the structure of benthonic fish assemblages. Results were spatially and temporally consistent suggesting that, in warmer temperate oceans, habitats dominated by species of smaller frondose brown algae can also play an important role in the structure and functioning of subtidal communities and may, to a certain extent, be considered analogous to other well-known vegetated habitats around the world (i.e. kelp forests, seagrass beds).

Biomanipulation of Furnas Lake, Azores: effects of repeated fish removal

Fumas Lake is a small mesoto eutrophic water body located in an ancient volcanic crater ofSiio Miguellsland Azores. In addition to agricultura( nutrient inputs, high densities of herbivorous fish (mainly common carp), as well as (irregular) hypolimnetic aeration contribute to maintain high levels of turbidity in the lake through resuspension of the nutrients accumulated in the sediment. ScHEFFER et al. ( 1993) suggested that turbidity in shallow lakes is not a smooth function of their nutrient status. Lakes are thought to have 2 distinct equilibria: a clear state dominated by macrophytes anda turbid state dominated by algae. A shift from turbid to clear water conditions is favoured by ( 1) reduction ofnutrient concentrations, reducing algal growth; (2) increased light availability, favouring vegetation, which retains sediments, protects zooplankton and consumes nutrients; and (3) reduction of planktivorous and benthos-feeding fish stock, reducing consumption of zooplankton, which grazes on phytoplankton, and reducing sediment resuspension with release of nutrients from the sediment pool (JEPPESEN et al. 2007). To improve water quality in the Fumas lake, a substantial part of the herbivorous and bottomfeeding (thus sediment resuspending) fish population, mainly common carp, was removed. Water quality was analysed in terms of turbidity, chlorophyll a, trophic state index values, macrophytes and zooplankton.