Understanding the interactions between salt marsh sediments and microphytobenthos successions as support for substrate structure

DA RODDA, C; PARODI, E.R.

Bariloche

Congreso; IV Congreso Latinoamericano de Sedimentología; 2005

Asociación Latinoamericana de sedimentología

The stability of cohesive sediments has been shown to be dependent on a number of physical factors. It can also be significantly modified by biological mediation (e.g. Paterson, 1997), i.e., macrofaunal species, microphytobenthos, and bacteria. Microphytobenthos is an important source of primary production in intertidal ecosystems (Admiraal, 1984; Underwood & Kromkamp, 1999) and epipelic diatoms dominate the community in fine and cohesive sediments (Admiraal, 1984; Thornton et al., 2002). The production of extracellular polymeric substances (EPS) during diatom locomotion (e.g. Hoagland et al., 1993; Wetherbee et al., 1998) will often form a smooth and stable sediment surface called biofilm (e.g. Paterson, 1997) that also stabilize the sediment surface (e.g. Paterson 1989; Yallop et al., 2000). This study aimed to understand the interactions between salt marsh sediments and microphytobenthos successions, and the assemblage of the substrate structure from a muddy salt marsh in the Bah¨ªa Blanca estuary (Argentina). The sampling area included 3 sets of tests composed by 2 squares of 1 m2 each: one of these (square a) without the first 5 cm of sediment surface, and the other square (square b) as a testing. Each month and at each sampling site, the microphytobenthic assemblage was sampled at low tide by scrapping the upper 1 cm of the sediment, and was analyzed with light microscope (LM) and scanning electron microscope (SEM). The early assemblage structure was described by characterizing the substrate and by estimating both relative abundance and contribution of each type and sized species. The biofilms showed a growth in thickness for the first month by aggregation and consolidation. The settlement of organisms increased with increased exposure time. A temporal analysis indicated that the epipelic assemblage was relatively homogeneous along the sampling area, but emphasized the specific succession of diatom species. At the species level, different arrangements of species coexistence and dominance were observed. After a month, the results showed an assemblage structure characterized by the presence of big size (~ 30 ¦Ìm) non-cohesive grains of slime, and numerically dominated by diatoms with larger surface:volume ratio, as well as their EPS filaments. After two months, the epipelic assemblage was mainly dominated by diatoms with low surface:volume ratio, with an increase in the number of cyanobacteria. Describing the epipelic assemblage using cell morphometry emphasized the contribution of large species, and revealed that the assemblage contained two diatom fractions, characterized by different behaviors, which may alternatively represent a large proportion of the biomass. The concomitant increment of biologic activity promotes the substrate consolidation, because particles and microorganisms were aggregated by abundant production of EPS. The temporal and spatial variation of natural biological communities must be interacted with physical conditions. The biostabilization is an important factor at Bahía Blanca estuary. Interdisciplinary work has revealed some associations, including that between the action of microphytobenthos and the sediment stabilization, and has been essential to examine the ecology of benthic ecosystems in relation to the other environmental factors in a complex web.