Microplankton metabolism along a trophic gradient in a shallow-temperate estuary

Abstract

Microplankton metabolism was studied in the Urdaibai estuary from June 1996 to May 1997 to investigate the factors that control the autotrophic-heterotrophic balance in three zones of contrasting trophic status. Gross primary production (GPP) and dark oxygen consumption (R) were measured from the upper eutrophic reaches to the lower marine segment for unfractionated samples as well as for nanoplankton ( 1 only during peak values of production, whilst in the middle estuary they were almost always ≥1. Ultraplankton was generally heterotrophic in the three zones. The relative importance of the <5 μm fraction increased seaward for both GPP and R rates. This size class was responsible for most of the oxygen consumption in the middle and lower estuary, while in the upper estuary the 5–20 μm size fraction was also relevant. During periods of enhanced phytoplankton growth, less than 20% of the total GPP was instantaneously respired in the three segments. In periods between peaks, only in the middle estuary was some percentage of the GPP not instantaneously processed. The percentage of the fractionated primary production respired in each size fraction was highly variable, although the highest values were found in the <5 μm for all stations.