Biological oceanography across the Southern Indian Ocean – basin scale trends in the zooplankton community

We present a study on the protozooplankton >5 μm and copepods larger than 50 μm at a series of contrasting stations across the Southern Indian Ocean (SIO). Numerically, over 80% of the copepod community across the transect was less than 650 μm in size, dominated by nauplii, and smaller copepods, while 80% of the biomass (as mg C m−3) was larger than 1300 μm in body length. Predation by the carnivorous copepod Corycaeus sp. was estimated to be able to remove up to 2% d−1 of the copepods <1000 μm in size. By the help of grazing models we estimated that primary producers were mainly grazed upon by ciliates and heterotrophic dinoflagellates (40–80% d−1 combined) in temperate waters but appendicularians became increasingly important in the tropical waters grazing about 40% of the biomass per day. Despite their high abundance and biomass, copepods contributed less than 20% of the grazing at most stations. Secondary production was low (carbon specific egg production <0.14 d−1) but typical for food limited oligotrophic oceans.

► 80% of the copepod abundance was <650 μm in size while 50% of the biomass was >300 μm.
► Ciliates were the major consumers on the primary production.
► Grazing impact by appendicularians increased towards tropical waters.
► Low specific production rates by copepods resulted in low trophic impact.
► The carnivorous copepod Corycaeus could remove 2% d−1 of <1000 μm sized zooplankton.