Seasonal and short-time-scale dynamics of microplankton community production and respiration in an inshore upwelling system

Abstract

An intensive study of pelagic primary production and microplankton community respiration was carried out during an entire upwelling season in the Ria de Vigo (NW Spain). From April to November measurements of oxygen production and respiration using the light-dark bottle technique were made twice a week at the surface, 1% light depth (1% LD, 12 ± 4 m) and 40 m (8 m above sea floor) alongside routine physical, chemical and biological measurements. During the major part of the survey period intermittent intrusions of cold, nutrient-rich upwelled water were observed in the ria with a periodicity of about 2 wk. Rates of gross primary production (GPP) were high but variable averaging 37.3 ± 30.7 µM O2 d-1 and 3.6 ± 4.8 µM O2 d-1 at the surface and 1% LD respectively over the period of survey (n = 50). Rates of dark community respiration (DCR) were also high and variable with maximum values being observed in the surface layer where the seasonal average was 12.2 ± 9.8 µM O2 d-1. At the 1% LD and 40 m, DCR averaged 5.3 ± 4.4 and 2.8 ± 3.0 µM O2 d-1 respectively. Although seasonal average and maximal DCR (up to 46.5 µM O2 d-1) were among the highest reported for coastal areas, microplankton production over the period of survey was dominated by autotrophic processes. Respiration losses by the microplankton community in the euphotic zone represented on average 43% of estimated mean seasonal water column GPP (2.1 to 2.7 g C m-2 d-1). Net heterotrophy in the aphotic layer consumed the equivalent of a further 25% of estimated water column GPP. The degree of coupling between primary production and respiration was primarily controlled by upwelling. During upwelling events respiration was generally low in the water column but it increased as a linear function of chlorophyll a concentration (R2 = 0.55, n = 13) and GPP (R2 = 0.47, n = 13) in the surface layer. Under such condition phytoplankton appears as the dominant component of community respiration consuming 14% of GPP. During periods of upwelling relaxation respiration was high relative to GPP. High water column respiration rates extending occasionally down to 40 m took place at the expense of organic matter trapped inside the bay. The seasonal breakdown of thermal stratification in autumn presented a relationship between surface respiration and chlorophyll a or GPP similar to that observed during upwelling events. The large excess primary production during this period was not remineralised inside the ria, suggesting that a large fraction may be exported towards the shelf.