Competitive organization of the soft-bottom macrobenthic communities of southern California lagoons

Abstract

The sandy-bottom macrobenthic community of Mugu Lagoon, a relatively pristine southern California (USA) marine lagoon, demonstrated (1) nearly constant community composition over 37 months of observation, (2) relatively little temporal variability in the population densities of the most abundant species over 37 months, and (3) a pattern of depth stratification in which very little vertical overlap existed among the six most abundant species. The only two species whose vertical distributions overlapped broadly showed horizontal spatial segregation, each abundant in different areas within the sand habitat. These community characteristics imply the importance of biological factors in structuring the sand benthos. The relatively large volume required for living space by these macrofauna suggests that competition for space may be the biological factor most important in determining the observed temporal and spatial abundance patterns. The muddy-sand community and the mud community of Mugu Lagoon also revealed similar patterns of stratification: new abundant species replaced species at the same sedimentary level while not greatly affecting species populations at other non-overlapping levels. In the sand community of Tijuana Slough, two of the abundant species of Mugu Lagoon's sand community were nearly absent as an apparent result of human over-exploitation. Probably in response, densities of species living at the sedimentary levels normally occupied by the missing species were much higher than would be predicted if competition for space were unimportant. In field experiments, removal of the deposit feeder Callianassa californiensis resulted in high recruitment of Sanguinolaria nuttallii, whereas control areas showed no S. nuttallii recruitment. Experiments also suggest that negative intraspecific interactions between Cryptomya californica individuals may explain the observed rapid emigration from areas of artificially high density. Perhaps the relatively great environmental predictability of southern California lagoons has permitted competitive interactions to play a singnificant role in determining the temporal and spatial abundance patterns of the soft-bottom macrobenthos.