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 fre quency and duration of sampling (ca twice per week from September 1990 to M ay 1992) are appropriate to reveal the impact of disturbances in the contex t of the seasonal cycle. Perturbations were identified statistically and re lated to recognisable meteorological and hydrographic processes that are ex pected to influence microplankton dynamics. According to the frequency of d isturbances (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 prom oted 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 ch anges in community structure: 15 out of the 21 disjunctions (ca 70%) coinci ded with thermohaline disturbances. Disturbances of the nutrient fields wer e related to allogenic forcing but also, markedly during the productive sea son, to re-structuring of the microplankton community. They pointed out aut ogenic community-driven processes. Microplankton assemblages revealed by th e species-oriented ordination technique were interpreted in terms of the re plication strategies: well-defined groups of competitor (C), disturbance-to lerant ruderal (R) and stress-tolerant (S) species were discerned. At a sea sonal time-scale, succession progressed according to the sequence from r- t o K-selection strategies. C-species dominated spring assemblages, during th e period of transient thermoclines, while S-species contributed more to sum mer assemblages, when the thermocline was fully developed and the ecosystem was under nutrient stress. R-species showed an intermittent pattern and th eir relative abundance increased when environmental variability was more ac ute, during the winter and spring to summer (clear-water phase) transition. The regime of allogenic disturbances operating at intermediate frequency d uring the productive season allowed the maintenance of relatively high micr oplankton diversity at the functional (assemblage) level: all types of repl ication 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.