Scale-dependent patterns of macrofaunal distribution in soft-sediment intertidal habitats along a large-scale estuarine gradient

Abstract

We investigated the pattern of distribution of intertidal soft-bottom fauna in streams and lagoons of the Uruguayan coast at three spatial scales. The Rı´o de la Plata and the Atlantic Ocean produce on this coast a large-scale gradient in salinity, defining a freshwater (west), an estuarine (central) and a marine (east) region. Within each region, there are several streams and coastal lagoons (sites) that define a second scale of variability. A third scale is given by intertidal gradients within each site. Species richness and total abundance was low in the freshwater west region and high in the central and east regions. The community in the west region was characterized by the clam Curbicula fluminea; in the other regions, it was dominated mainly by the polychaete Heteromastus similis. The polychaete Nephtys fluviatilis was more abundant in the east region, while another polychaete, Laeonereis acuta, characterized the central region. Sediment fractions did not vary significantly at this scale. At the scale of the sites, species richness and total macrofaunal abundance were higher in coastal lagoons than in streams. Coarse sands were more common in coastal lagoons, while medium and fine sand characterized the sediment in streams. Within each site, species richness and total abundance increased towards the lower intertidal level; the macrofauna of the upper levels were a subsample of the fauna occurring at the lower levels. There was also a significantly lower proportion of fine sand at the upper level. At regional scales, the observed patterns may be indirectly or directly related to the gradient in salinity, through differential physiological tolerance to osmotic stress. At the scale of the sites, variability may be explained mainly by geomorphological and sedimentological differences between lagoons and streams. Variation among levels may be related to gradients in desiccation, colonization and predation.

Hydrologic interactions of infaunal polychaetes and intertidal groundwater discharge

“…In an intertidal groundwater seep zone, porewater salinity can range from 1 to 28 in several meters of horizontal distance across a sand flat (Dale & Miller 2007), and, over time, porewater salinity at a single point can vary from 2.6 to 28.5 seasonally (Miller & Ullman 2004). Thus, groundwater discharge creates spatial and temporal variability in porewater salinity, with magnitudes similar to those well-known to affect benthic organisms in estuaries (Holland et al 1987, Jorcin 1999, Ysebaert et al 2003, Gimenez et al 2005. In contrast to the typical estuarine salinity gradients over 1 to 100 km, however, the spatial changes in salinity at our study site at Cape Henlopen, Delaware, USA, occur on scales of 1 to 100 m.…”

“…Large-scale studies of communities along estuarine gradients show a positive correlation between biodiversity and salinity (Holland et al 1987, Jorcin 1999, Ysebaert et al 2003, Gimenez et al 2005. At a single site, species distribution can be affected by environmental factors such as grain size (Fernandes & Soares-Gomes 2006), rate of sedimentation (Anderson et al 2004), beach elevation and morphology (McLachlan & Jaramillo 1995), and aperiodic weather-driven tidal events (Brazeiro & Defeo 1996).…”

Temporal dynamics of deep infralittoral macrobenthic fauna in a subtropical estuarine environment

“…Low salinity and/or its variability are known to be responsible for a consistently low number of species occurring in estuarine waters when compared to adjacent environments (Giménez et al, 2005). Density reductions or even absence of marine species inside the estuarine region from winter to spring 2000 were associated to the lower salinity and almost limnetic conditions registered along these seasons.…”

“…Inside estuarine regions, sediment characteristics of the substrate (e.g. grain size, organic matter content) and in particular salinity of the water column have long been reported as the main abiotic variables which can determine the spatial-temporal patterns of distribution on macrobenthic assemblages (Holland et al, 1987;Warwick et al, 1991;Schlacher & Wooldridge, 1996;Mannino & Montagna, 1997;Giménez et al, 2005;Cortelezzi et al, 2007).…”

Benthic communities on hard substrates covered by Limnoperna fortunei Dunker (Bivalvia, Mytilidae) at an estuarine beach (Río de la Plata, Argentina)

“…Benthic ecological studies are difficult, scarce, and fragmented because of the extensive physical dimensions of such a system, and as a result of limited financial resource as well. Nevertheless, some studies have been carried out on the seasonal dynamics and spatial-distribution patterns of the macroinvertebrate fauna along both banks of the estuary as well as within the surf zone (Boschi 1988;Rodrigues Capítulo et al 2003;Giberto et al 2004;Venturini et al 2004;Giménez et al 2005;Cortelezzi et al 2007;Ocón et al 2008). Studies, however, on the hard littoral substrates of the benthos (i.e., especially those on the benthic epifauna) in the Río de la Plata Estuary have been notably scarce (e.g., Darrigran et al 1998).…”