Effect of meteorological and hydrographic disturbances on the microplankton community structure in the Ria de Vigo (NW Spain)

The relationships between meteorological and hydrographic disturbances and microplankton community structure and organisation were investigated in the Ria de Vigo, an estuarine ecosystem affected by coastal upwelling. The frequency and duration of sampling (ca twice per week from September 1990 to May 1992) are appropriate to reveal the impact of disturbances in the context of the seasonal cycle. Perturbations were identified statistically and related to recognisable meteorological and hydrographic processes that are expected to influence microplankton dynamics. According to the frequency of disturbances (between 8 and 14 yr(-1)) the ecosystem could be classified as intermediate-frequency disturbed in relation to the microplankton community organisation timescales. Meteorological and thermohaline disturbances promoted changes in microplankton community structure (disjunctions). The time-series of thermohaline properties, a proxy for allogenic processes related with mixing regimes and estuarine circulation, was the best indicator of changes in community structure: 15 out of the 21 disjunctions (ca 70%) coincided with thermohaline disturbances. Disturbances of the nutrient fields were related to allogenic forcing but also, markedly during the productive season, to re-structuring of the microplankton community. They pointed out autogenic community-driven processes. Microplankton assemblages revealed by the species-oriented ordination technique were interpreted in terms of the replication strategies: well-defined groups of competitor (C), disturbance-tolerant ruderal (R) and stress-tolerant (S) species were discerned. At a seasonal time-scale, succession progressed according to the sequence from r- to K-selection strategies. C-species dominated spring assemblages, during the period of transient thermoclines, while S-species contributed more to summer assemblages, when the thermocline was fully developed and the ecosystem was under nutrient stress. R-species showed an intermittent pattern and their relative abundance increased when environmental variability was more acute, during the winter and spring to summer (clear-water phase) transition. The regime of allogenic disturbances operating at intermediate frequency during the productive season allowed the maintenance of relatively high microplankton diversity at the functional (assemblage) level: all types of replication strategies were present during summer and early autumn. The results support the applicability of the intermediate disturbance hypothesis (IDH) to microplankton community structure in the Ria de Vigo.