Mesoscale estimations of primary production in shelf waters: a case study in the Golfo Artabro (NW Spain)

Abstract

Primary production was estimated in an area of ∼500 nm 2 (1645 km 2) in the Golfo Artabro (Galicia, NW Spain) during March 1993. Estimations were made either using vertical profiles of simulated `in situ' incubations of water samples, determination of photosynthetic parameters at the surface and the bottom of the euphotic zone (100% and 1% of surface irradiance, respectively) and estimations made entirely from variables measured at the sea surface. Complementary measurements included vertical profiles of temperature, salinity, irradiance and chlorophyll- a concentration at 38 stations regularly distributed over the continental shelf, and phytoplankton species composition at 10 stations. The study area was divided in six zones according to the hydrography and chlorophyll concentration, each zone including at least one station in which vertical profiles of primary production and photosynthetic parameters were measured. A synoptic map of water column integrated estimations of daily primary production was computed using average values of photosynthetic parameters in each zone and the measured chlorophyll concentrations. The observed situation corresponded to a period between spring blooms, in which a previous bloom dominated by the diatom Chaetoceros socialis was disrupted by a change in the meteorological conditions and the intrusion of a saline water body into the shelf. The saline intrusion caused the formation of a thermohaline front in mid shelf, separating slightly stratified outer shelf waters from mixed inner shelf waters. A new bloom started in the outer shelf side of the front and also in some locations near the coast, while the mid shelf area had the lowest chlorophyll concentrations and primary production rates. Maximum values of primary production were equivalent to those measured during upwelling events although non-upwelling conditions prevailed during the study. Meanwhile phytoplankton species composition was relatively uniform over the shelf. This study illustrates the feasibility of mapping primary production in relatively large shelf areas using photosynthetic parameters and vertical profiles of irradiance and chlorophyll in selected subareas. The employed method can be easily combined with remotely sensed data to produce large scale estimations. However, parallel estimations of primary production with other methods are needed to obtain appropriate interconversion factors and allow for comparison of production values between different datasets or computation models.