Benthic diversity and assemblage structure of a north Patagonian rocky shore: a monitoring legacy of the NaGISA project

Abstract

The rocky shore of Punta Este, Golfo Nuevo (Patagonia, Argentina), was sampled using the standardized NaGISA (CoML) protocol aimed to generate biodiversity baseline data in 6 levels, from high intertidal to 10m depth. Based on the generated data, we studied the benthic assemblage structure, species richness, mean abundance and the distribution pattern of invertebrate functional groups, typifying species in each intertidal and subtidal level. The rocky shore studied, as other in north Patagonia is understudied, factors driven its assemblage structure are not clear yet. The intertidal sampled is exposed to extreme physical conditions higher than any other studied rocky shore systems, with air temperature variation of 40°C during the year, maximum winds of 90 km/h and semidiurnal tides of 5m amplitude; on the other hand subtidal presents less thermal variation (DeltaT 10°C along the year) and more homogeneous physical conditions. We identified 65 taxa represented by six animal phyla: Mollusca, Arthropoda, Annelida (Polychaeta), Echinodermata, Cnidaria and Nemertea; and three algal phyla: Chlorophyta, Rhodophyta and Heterokontophyta (class: Phaeophyceae). Ordination nMDS plots showed three different assemblages in terms of species composition (intertidal, subtidal 1m level and subtidal 5-10m samples). The intertidal was represented by suspension feeders mainly Mollusca. The tiny mussels Brachidontes rodriguezii and Brachidontes purpuratus, and the algae Corallina officinalis dominated the intertidal and acts as a buffer that prevent other species for physical stress. The subtidal was mainly represented by grazers. Our results showed a tendency of suspension feeders decreasing and increasing of grazers and predators from high intertidal to subtidal, probably driven by decreasing in physical stress. The gastropod Tegula patagonica, the sea urchins Arbacia dufresnii and Pseudechinus magellanicus and the non native algae Undaria pinnatifida were the most abundant in 1m-10m level. Based on previous works performed in the region, we hypothesize that the differences registered between intertidal and subtidal samples could be explained in part by an increase in physical stress in the intertidal with low predation pressure that promotes positive interactions, while in the subtidal the increasing in consumers and decreasing of physical stress could lead to associational defenses. Competition for primary substrate at the intertidal and subtidal must be explored in future experiments. Our results could be useful to compare data and to develop a sustainable network for long-term monitoring benthic community changes due to anthropogenic activities.