by Sá Rodrigues, Pedro de

Abstract (Summary)
The present study aims to analyze the structure and dynamics of the intertidal macrobenthic assemblages in two sites with distinct environmental characteristics of Cassino beach, RS, Brazil. Site 1 can be characterized as an extreme dissipative beach, with fine sediments and low hydrodynamics and site 2 is a dissipative/intermediate beach, with coarser sands and high hydrodynamics. Sporadic muddy deposition events occur in site 1. Sampling was conduct monthly from april/2004 to may/2005. Data from the fist two months was considered as a pilot study and was included just in chapter III. In each site, three transect lines, spaced 50 m apart, were defined perpendicular to the shore line, samples were collect in triplicate in 4 levels (10 m apart). Each transect wasextend from 10 above de swash limit to 1m deep in the inner surf zone. Chapter I aimed to describe the temporal variability of the intertidal assemblages. A striking dynamics was recognized through samplings times. The mains temporal changes were influenced by the population dynamics of numerically dominants species and physical disturbances. In winter was found a low density and diversity of the assemblages, with were related to absence of recruitments, an event of muddy deposition and stranding events of low effects. In spring was observed a population explosion of the polychaeta Scolelepis gaucha, migration of Mesodesma mactroides adults from the subtidal to the intertidal and a strand event. During summer was observed recruitments of tidal migrant species as M. mactoides, Donax hanleyanus and Emerita brasiliensis. Fall was characterized by low densities, except for an expressive recruitment of D. hanleyanus.Chapter 2 aimed to describe the spatial distribution across shore and along shore in distinct scales. Site 2 showed a higher diversity than site 1. The higher diversity wereprobably due the presence of amphipods Phoxocephalopsis sp. and Plastyschnopidae in site 2, which were absent in site 1. The results showed that dissipative/intermediate can9 support higher diversities than extreme dissipative conditions, probably because the assemblage can be beneficiated by the higher hydrodynamics of swash. The lowdiversities found in site 1, may be also related to sporadic muddy deposition events. The multivariate analyses showed three macrofaunal zones: mesolitoral; the higher swash;the lower swash together with the inner surf zone. The characteristics species of each zone varied according to the season. A decrease of density was verified from mesolitoral zone to the lower levels. No patterns were identified in a smaller scale (50m) of the along shore distribution, probably due the high homogeneity of the morpho-dynamics and sediments in this scale. In Chapter III, the period that occurred two mortality events were analyzed with more detail: muddy deposition in april/2004 and stranding in october/2004. The recuperation of the macrofauna assemblages was extremely fast, in three months they showed the same composition, although the populations were characterized by high densities of recruitments and juvenis. The adult stokes may be heavily impacted by this events. The muddy deposition caused a mortality of all population of the intertidal assemblages. The bivalve M. mactroides juvenis+adults was the organism most affected by the strand event. It?s likely that these events are the main long term factors structuring this assemblage.
This document abstract is also available in Portuguese.
Bibliographical Information:

Advisor:Sergio Antonio Netto; César Serra Bonifácio Costa; José Guilherme Bersano Filho; Carlos Emílio Bemvenuti

School:Fundação Universidade Federal do Rio Grande

School Location:Brazil

Source Type:Master's Thesis

Keywords:Macrofauna bentônica embancamento Benthic macrofauna sandy beaches temporal variability spatial muddy deposition stranding


Date of Publication:03/24/2006

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