The seasonal abundance and production of Oithona nana (Copepoda:Cyclopoida) in Southampton Water

Abstract

Recent studies indicate that Oithona spp. contribute significantly to total copepod biomass. Little is known, however, about their ecological significance, particularly in the case of the estuarine Oithona nana. A study comprising three sites within Southampton Water was conducted to evaluate the late-stage copepodite/adult (stages IV–VI) O. nana community, using 120-mm mesh nets. Although present throughout the estuary, there was a striking spatial gradient with O. nana most common in the upper estuary. A clear seasonal pattern was observed with O. nana as the most abundant copepod species from late summer until early winter. It comprised 61% of all copepods recorded, with a biomass of 757.22 mg C m–3. Production estimates of O. nana were derived from the ‘instantaneous-growth’ approach, using appropriate growth equations. The estimated production of O. nana ranged from 1.50 mg C m–3 year–1 within the lower estuary to 146.77 mg C m–3 year–1 in the upper estuary. In the upper estuary, this compares with production rates of 187.47 mg C m–3 year–1 for all Acartia congeners (excluding nauplii), the most common calanoid genus. Throughout the estuary, O. nana annual production represented 18% of total copepod production clearly indicating that, at least in the upper estuary, O. nana production may be directly comparable with calanoid production.