Seasonal and inter-annual changes in the planktonic communities of the northwest Alboran Sea (Mediterranean Sea)

The response of the northwestern Alboran Sea pelagic ecosystem to temporal changes in hydrological conditions has been examined for the time period of 1992–2002. In addition, the bottom–up linkages between the lower trophic levels and the growth and nutritional status of sardine larvae were examined using quarterly data from 1992 to 2002 within the frame of the monitoring Project ECOMALAGA. The study area was characterised by the almost permanent presence of an upwelling which was intensified in the spring period. Consequently, an annual peak of nutrients was usually found during this season when the nitrate concentration averaged 1.35 μM. Accordingly, chlorophyll-a concentration and cell abundance of micro- plus nano-phytoplankton increased in that season (1.51 μg L−1 and 446 cell mL−1 compared to 0.85 μg L−1 and 225 cell mL−1 obtained from summer to fall). Despite these seasonal changes, the analysis of the taxonomic composition of the phytoplankton communities did not reveal a clear annual succession pattern. Contrastingly, peaks of zooplankton abundance were obtained in summer (1964 ind m−3) due to the increased presence of brachiopods with respect to copepods (which dominated from fall to spring). Significant inter-annual changes were obtained in the phytoplankton and zooplankton communities. Thus, dinoflagellate and coccolitophorid abundances relative to diatom abundances tended to increase from 1997 to 2002. This trend matched the progressive reduction of the upwelling intensity. These inter-annual changes significantly affected the larval growth of Sardine pilchardus and their nutritional condition, as higher growth rates in terms of body length coupled to higher somatic mass increases (expressed by DNA content) occurred in spring, matching with the higher chlorophyll-a concentration. Furthermore, the highest larval growth was obtained in 2001, coinciding with the change observed in the composition of phytoplankton community.