Spatio-Temporal Variability in the Structure of Benthic Populations in a Physically Controlled System off Terra Nova Bay: The Shallow Hard Bottoms

Abstract

Along the coast off Terra Nova Bay (Ross Sea), the shallow hard bottoms are subjected to high levels of physical disturbance due to sea-ice scouring, and are characterized by a dense covering of two red macroalgae, Iridaea cordata and Phyllophora antarctica. These algae show a typical distribution, with I. cordata occurring at shallower depths (2–15m) and P. antarctica progressively replacing the former species in deeper waters (5–35 m). Several vagile invertebrate species often occur often in relatively high numbers associated to these algae. The aim of this chapter is to give an overview of the spatial variability in the structure and distribution of the benthic communities of this peculiar biotope of the bay and to provide a preliminary evaluation of the interannual variability in some structural parameters in relation to the environment. Most of the samples were collected by SCUBA diving (scraping fixed surfaces of 40 × 40 cm) in seven different sites located around the Italian base during the three summer Antarctic expeditions in 1989/90, 1993/94 and 1994/95.

More than 83 000 specimens of macrobenthic organisms were collected, belonging to 62 taxa; the most diversified and abundant groups were polychaetes (19 taxa, 4.2% of the total), molluscs (11 taxa, 56.6%), amphipods (14 taxa, 10%), tanaids (2 taxa, 21.6%) and isopods (7 taxa, 7.5%). The most abundant species, that can be considered the core of the community, were Harmothoe spp. (mainly belonging to two forms of H. spinosa), Pionosyllis comosa, Kefersteinia fauveli and Ophryotrocha claparedii (polychaetes), Laevilitorina antarctica, Powellisetia deserta, Onoba gelida and O. turqueti, and Philobrya sublaevis (molluscs), Nototanais dimorphus (crustacean tanaids), Munna antarctica, Paramunna rostrata and Austrosignum sp. (isopods), Oradarea acuminata, O. rossii, Paramoera walkeri and Prostebbingia serrata (amphipods), and Sterechinus neumayeri and Odontastervalidus (echinoderms). A great homogeneity was recorded in the various sites examined with regards to species composition. The main differences among stations were due to the relative abundance of common species and to the occurrence of quite rare ones. Correlations (Spearman rank correlation) among the community structural variables and depth showed that both the Iridaea and the total algal biomass decreased significantly (P < 0.05) depending on the depth, whereas both the number of animal taxa and the diversity (H′) increased with depth (P < 0.01). Animal density increased with increasing Phyllophora as well as total algal biomass, while diversity H′ decreased according to the same variables. Distribution at the community and species levels revealed a well-defined zonation pattern with depth, covering and architecture of the two dominant macroalgae. Species richness and diversity were higher in the Phyllophora-associated community, where habitat complexity and shelter, due to both algal architecture and reduced environmental stress, were higher. The highest faunal abundance (over 82000 ind m⁻²) was observed in association with the Iridaea covering, where the harsher environmental conditions selected only few taxa. Inter-annual differences in algal biomass and animal density were recorded in the few stations where we could compare three years of observations. The samples at site Faraglione showed the strongest fluctuations with a remarkable reduction in animal diversity and density (almost one order of magnitude) between the summers of 1990 and 1994, followed by relatively high values again in 1995. These interannual differences, more pronounced at the shallower stations (2–4 m depth) than at the deeper ones, could be related to variability in sea-ice development and persistence. As regards sea-ice conditions, in summer 1994 there was a delay of about 40 days in the melting of the seasonal fast- and pack ice in Terra Nova Bay. Notwithstanding these numerical changes, the basic structure of the community was maintained over time, probably due to the high motility potential of the species and to their reproductive strategies.