Impact of wave climate of massive mortality events of polititapes rhomboides clams in an upwelling-driven bay

Abstract

Subtidal natural beds of Polititapes rhomboides in the Ría de Vigo (NW Iberia coastal upwelling system) suffered massive mortality events in June 2010 (Darriba et al., 2019). Abnormal high-intensity infections of intracellular prokaryotic colonies (Rickettsiales-like) in the gills, observed in the histopathological study, could explain the clam’s mortality and the drop in this wild population. An atypical long period (7 months) of continuous and strong wave storms entailed high bottom boundary layer dynamics with intense sediment mobilization and resuspension. Immediately after this energetic period, warm water (17ºC) conditions near the seabed prevailed, and these could promote prokaryote colonies proliferation. Stress produced by the continuous wave impact on the seabed could have weakened the clam’s ecophysiology inhibiting an efficient immune response. This physiological status would coincide with the expansion and infection of Rickettsial colonies in the gills, probably boosted by the warmer temperatures. This synergetic effect would have produced massive clam mortality events. The analysis of wave climate winter variability shows an evident interannual variability related to the NAO (North Atlantic Oscillation) and WEPA (West Europe Pressure Anomaly) climate indices (Villacieros-Robineau et al., 2021). Winters with the highest waves occur during the coincidence of negative NAO and positive WEPA phases. Warmer temperatures and low salinity conditions (downwelling scenarios) are more frequent during these severe winters. Collapses of banded carpet shell populations, with a critical reduction in the abundance and a production decline, occurred after these energetic winters (2000/2001, 2009/2010). During intermediate recovery periods (years with low energy conditions, e.g., 2006-2009), clam populations grow until the subsequent collapse. This multidisciplinary approach, including histopathological, oceanographic, and population ecology studies, shows that monitoring environmental stressors related to the climate indices can be used as an ecological indicator of the clam’s health population status and evolution with potential applications for managing this and other marine resources