Impact of the seasonal and interannual oceanographic variation on the assemblages of microphytoplankton, mesozooplankton, ichthyoplankton and macrozoobenthos in two coastal areas of northern Perú from 1994 to 2002

Abstract

It was analyzed the oceanographic variation and the responses of microphytoplankton, mesozooplankton, ichthyoplancton and macrobenthos assemblages in the coastal areas (< 20 nm) off Paita (05°S) and off San José (06°45’S) from 1994 to 2002. The oceanographic variation was composed by different time-scales, which were modulated by the ENSO cycle, the poleward propagation of coastal trapped waves (CTWs) resulting from Kelvin waves impacting the coast near the Equator, and the seasonal intensification of coastal upwelling. The sequence of the events El Niño (EN) 1997-98 and La Niña (LN) 1998-99 showed welldefined characteristics in the surface physical conditions and in the vertical structure of the water column.
Before the official onset of the EN 1997-98 event in May 1997, a Kelvin wave impacted the northern coast, triggering positive temperature anomalies, a deepening of the vertical structure and the presence of some biological indicators of warm water masses in February and in April 1997. These conditions prevailed or were intensified until the end of the event. The LN 1998-99 event was characterized by the absence of warm water masses near the coast and the dominance of cold coastal waters, the absence of Kelvin waves, the shallow position of cold, oxygen-poor waters, and the dominance of planktonic indicators of cold coastal waters. Seasonally, there was a tendency that more oxygenated subsurface conditions (a deeper oxycline) were present during autumm/winter, while less oxygenated subsurface conditions (hypoxic, a shallower oxycline) were developed during summer. This pattern does not agree with the upwelling seasonal regime, but coincides with the expected dynamics of the Southern Extension of the Cromwell Undercurrent at these latitudes. In most of the cases there was a good fit of the tolerance ranges of some planktonic species to the dominant water masses in the surface layer: Cold Coastal Waters (CCW), Surface Equatorial Waters (SEW) and Surface Subtropical Waters (SSW), validating the usefulness of these species as biological indicators of water masses. There were determined the tolerance ranges of temperature and salinity from the phytoplankters Protoperidinium obtusum (CCW), Ceratium breve (SEW) y Ceratium praelongum (SSW), and from the zooplankters Centropages brachiatus (CCW), Eucalanus inermis (CCW), Centropages furcatus (SEW), Mecynocera clausi (SSW), among others. The indicators varied their distribution along the study period. CCW indicators were found most of the times but not during the EN 1997-98 event; (a) in this pe-riod the dinoflagellate Goniodoma polyedricum reached its highest frequency off San José and the dinoflagellate Pyrocystis lunula peaked in frequency off Paita; (b) the abundance of the copepod Centropages furcatus (SEW) was increased off Paita, and was found off San José; (c) during the EN 1997-98 event the species composition of plankton was changed, increasing the
total number of species due to the input of non-resident species; (d) Phytoplankton and zooplankton volumes tended to show a direct relationship under neutral ENSO conditions, but showed a negative relationship under El Niño and La Niña events; (e) the macrobenthic communities were impacted negatively in abundance and diversity by the EN 1997-98 event in both areas, contrasting with the response of the benthic macrofauna to EN off the central Peruvian coast. This behaviour could respond to the disturbance of the sedimentary environment off Paita due to the highly enhanced discharges of the Chira river. Off San
José the impo-verishment of the macrobenthos may have resulted from the local reduction of primary production and of particulate flux to benthos. It is postulated that in this area the food limitation for benthos, due to erosive processes in the bottom, sets up a key difference of the dynamics of the benthic communities, compared to those off the central coast that inhabit areas of enhanced deposition and accumulation of particulate organic matter.
Gammarid amphipods, specially those belonging to the family Ampeliscidae, showed the most consistent response to the environmental variability, since the populations were significantly reduced during the EN 1997-98 event in both coastal areas.