Structure and production of the benthic macroinvertebrate community in a shallow lagoon in the Bay of Cadiz

The composition and distribution of the benthic macroinvertebrate community in a lagoon fish-pond system in the Bay of Cadiz. Spain, were studied by taking monthly samples at 4 sites during 1991. A total of 29 macroinvertebrate species, representing 16 orders. were identified. Gastropods, amphipods and chironomid larvae dominated the macrofauna in term of numbers of individuals, while in biomass terms gastropods, bivalves and polychaetes were dominant. The diversity (H') and evenness (J') indices of this community were low, ranging from 0.58 to 1.85 and 0.30 to 0.75 respectively (natural log base). Monthly total invertebrate density showed considerable seasonal variation, with a main peak in winter. Taking the most abundant species only, the densities of Hydrobja minoricensis, H. ventrosa, Microdeutopus gryllotalpa and Chironomus salinarius were positively related, and the density of Nereis diversicolor negatively related, to the biomass of benthic macroalgae (Ulva lactuca and Cladophora sp.). The density of C. salinarius was also positively related to the silt content in the sediment. Mean annual production varied between 13.3 g m-' yr-' ash-free dry weight (50.7 g m-2 yrr' dry weight) and 27.3 g m-' yr-' AFDW (122.0 g m-'yr-' DW) depending on the sampling site, yielding production/biomass ( P / B ) ratios between 1.08 and 1.37 yr-l. The annual production estimate for the whole system was 19.5 g m-' yr-' AFDW (66.6 g yr-' DW).