The discovery of global-scale latitudinal gradients of declining biodiversity from the tropics to the pole for bivalves, gastropods and isopods in the deep North Atlantic has created a high degree of interest and controversy. This is because such gradients are commonly associated with solar energytemperature gradients in terrestrial and shallow water systems and it is difficult to see how these processes might apply to a diversity gradient in the deep North Atlantic, where productivity increases northwards but diversity declines. Here, we compare biodversity patterns from marine nematodes, the most abundant deep-sea metazoan, from the deep North Atlantic with previous results and show that rarefaction is potentially unsuitable for large-scale biogeographic pattern analysis. We obtain a different pattern from that previously obtained for mollusc and isopod data. Nematode diversity, as measured by species count, shows a positive gradient between 13 to 56"N, which is consistent with the hypothesis that this pattern is related to the productivity gradient in the food-starved deep North Atlantic. The Norwegian Sea appears to be an area of low diversity for reasons connected to historicalgeographical processes.

The discovery of an apparently positive latitudinal gradient in nematode species richness over a limited geographic area in the North Atlantic, leading to the hypothesis that it is associated with a positive latitudinal organic flux gradient, has created some debate. A test of this hypothesis is that the negative latitudinal organic flux gradient in the central equatorial Pacific should lead to an associated negative gradient in species richness. Here, we show that species richness in the central equatorial Pacific is positively associated with the organic flux predicted from the pattern reported for the North Atlantic. The patterns in nematode species richness differ from other deep-sea organisms; they seem to be entirely related to modern ecology and unaffected by historical events.

No abstract

The distribution and abundance of shallow subtidal meiofauna in four habitats in the Niantic and Pettaquamscutt estuaries in southeastern New England were studied from October 1964 through October 1965. Numbers of individuals ranged from 1,184 to 5,163/10 cm, and wet weights from 8.5 to 62.5 mg/10 cm. Nematodes were the dominant group, averaging 83% of the total numbers and 64% of the total biomass. Among the nematodes, epigrowth-feeding species were dominant at all four stations; in addition, deposit-feeding species were abundant at three stations where the amount of detritus was high relative to the fourth station. At these three stations ostracods and deposit-feeding polychaetes were also abundant.Nematodes showed marked seasonal changes i their species composition, with epigrowth-feeding species reaching maximum densities in spring and summer (coincident with observed increases in benthic microflora production), and deposit and omnivorous-feeding species in fall and winter (coincident with observed increases in organic detritus). Increases in the epigrowth feeders were responsible for significant increases in the total nematode populations in spring and summer.Ostracods were most abundant in late summer, fall and winter in association with the increases in detritus. Polychaetes, amphipods and lamellibranchs were most abundant in summer, the first-named in association with elevated water temperatures. Harpactacoid copepods were inconsistent in their seasonal distributions.Approximately 80% of the meiofauna occurred in the upper 3 cm of sediment. Copepods and ostracods were virtually limited to the upper 3 cm, while amphipods and lamellibranchs were limited to the upper 2 cm of sediment. Nematodes and polychaetes extended down to 5 cm (the lowest depth studied), although usually in significantly reduced numbers.