Temporal and spatial variability in shallow (10-20 meters), tropical, soft bottom communities of Mamala Bay, Hawaii were examined. The three objectives of the study were: (1) to determine if temporal variability (lunar periodicity and seasonality) exists in community structure and biomass of Hawaiian soft bottom benthos; (2) to examine the temporal nature of the benthic response to a freshwater runoff event; (3) to examine spatial variability in Hawaiian soft bottom benthos and its relationship to freshwater runoff. Benthic samples were obtained monthly for a period of twenty-five months to examine seasonal variations. Stations were more intensively sampled for a three-month period (every ten days) to examine lunar periodicity and the impact of a large runoff event. Small scale spatial variability was examined by comparing samples from the crest and trough of sedimentary ripples and from sand and rubble. Larger scale spatial patterns were examined during the summer (August 1993) and winter (February 1994) by sampling seven equally-spaced stations along a 3-kilometer (km) transect on the 10 meter (m) isobath leading east from a source of freshwater runoff, the Ala Wai Canal. Three additional stations (20 m) were sampled in February 1994 to examine depth-related changes.

The benthic communities were dominated by a few groups with significant temporal variations in density. Temporal dominance was especially evident in polychaetes (Syllidae and Pisionidae). Macrofauna densities ranged from 4,910 to 47,425 individuals per square meter with biomass ranging from 47 to 1091 milligrams per square meter. The dominant taxa, density and biomass of soft bottom communities were consistent with data from other shallow tropical and subtropical areas. There was no evidence that large wave events or freshwater runoff influences the density or community composition on short time scales. Significant within-month fluctuations in the density of major taxonomic groups existed, with evidence of lunar periodicity in the arthropods (primarily ostracods). There were significant between-year differences in density of macrofauna; it is possible this reflects a pattern spanning several years related to the 1991-1994 ENSO event.

Although the shallow, wave-swept sedimentary environment would be expected to produce a relatively homogenous biological community associated with the homogenous shifting sediments, small scale (< 30 m) differences exist between crest and trough of sedimentary ripple bedforms and between sand and rubble areas. Specifically, higher densities of syllids and copepods were associated with the crest of ripples. Higher densities, taxonomic richness and biomass were present within rubble. On a larger scale, along a 3.0-km transect eastward from the Ala Wai Canal, communities are similar with regards to density, biomass and dominance by motile, detritivorous and omnivorous polychaetes and motile, brooding crustaceans. In contrast, taxonomic diversity differed along the transect, with sites located near the mouth of the Ala Wai Canal, a source of freshwater runoff, consistently more diverse than Waikiki sites. There was no evidence that runoff from the Ala Wai Canal had a negative impact or was an important structuring mechanism for adjacent soft bottom shallow communities. The lack of impact could be related to the type of estuary examined, where little of the particulate matter was exported out of the system.