Effects of a phytodetrital input on nematode communities of the abyssal, equatorial Pacific

Abstract

Sediment samples were collected at four sites along a latitudinal gradient of phytodetrital deposition from 0 to 9°N at 140°W in the central equatorial Pacific, as apart of the US Joint Global Ocean Flux Study, and at a reference site at 23°N, 158°W. Stations at 0-5°N were observed to receive an input of phytodetritus and elevated particulate organic carbon (POC) flux, whereas no phytodetritus was recorded at stations at 9 and 23°N and POC flux was lower. Nematode abundance, biomass, functional group composition and species diversity were quantified. These data were used to compare observed patterns of spatial distribution with the predictions of community ecology theory on two scales; small localised patchiness, and latitudinal gradients of change.

Increases in nematode abundance and total biomass were significantly correlated with POC flux (measured into deep-moored sediment traps) along the 140°W transect. Between 0 and 5°N, nematode abundance and biomass in the surface (0-1cm) sediment layer fell at the low end of the range recorded from other previously-studied sites in the NE Atlantic of comparable depth, that are known to receive an input of phytodetritus and elevated POC flux. Total nematode biomass was greatest at 5°N and was highly correlated with 5-month POC flux. It is hypothesised that this reflects the generation times of nematodes responding to POC flux and that previous studies that have failed to record a correlation between nematode biomass and POC flux may be explained by the use of response time-scales derived from cultures of shallow-water nematodes.

Within-habit (alpha) species diversity was greatest at the equatorial station and decreased with increasing latitude. All alpha diversity indices were significantly, positively correlated with annual POC flux, and quantity of phytodetritus overlying the sediment. Multivariate analysis of the species assemblages indicated that differences in community structure were best explained by annual POC flux.

Comparisons with other previously-studied sites in the NE Atlantic indicate that, at local scales, there is a characteristic fauna assemblage that can be identified as a 'phytodetrital' fauna, at least at the genus level.

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