Abstract
The temporal and spatial changes of zooplankton and chlorophyll a concentration were studied during the warm stratification period (early June) at three stations whose traits corresponded to the coastal, frontal, and offshore-dome water conditions described for the Catalan Sea. We sampled the stations for 12 days at a frequency ranging from less than 10 to 10 2 h, with a spatial resolution ranging from 10 to 10 4 m. The objective was to determine the variability of mesozooplankton and phytoplankton (chlorophyll a ) biomass, and average individual size (mass) across a coast-offshore transect in relation to the stratification conditions prevailing in the NW Mediterranean during summer. The vertical distribution of phytoplankton biomass displayed a clear deep maximum at 60 m depth except at the coastal station. This maximum exists during most of the year and is especially important during the density stratification period. It was accompanied during daylight hours by a coherent zooplankton maximum. At sunset mesozooplankton ascended and dispersed, with larger organisms from deeper layers joining the migrating community and increasing the average individual mass. The highest variability of mesozooplankton biomass, individual mass and chlorophyll a concentration occurred at the front station due to the coupling between the vertical migration of zooplankton and the particular characteristics of the front. According to the data shown, the highest variability was observed at the lowest scales.
Highlights
The study of the effects of physical singularities on the characterisitics of plankton distribution, structure and function in the Catalan Sea started about four decades ago, in the framework of a series of research projects led by Dr Marta Estrada
The study was done in a 120-km transect approximately perpendicular to the coastline (Fig. 1), running from Barcelona to the middle of the Majorca–Menorca channel
Of late spring-early summer in the NW Mediterranean (Estrada 1985, Estrada and Margalef 1988, Saiz and Alcaraz 1990), with the signature of the front occluded at surface by the mixing layer above the thermocline
Summary
The study of the effects of physical singularities (i.e., mesoscale features and density stratification) on the characterisitics of plankton distribution, structure and function in the Catalan Sea started about four decades ago, in the framework of a series of research projects led by Dr Marta Estrada (see Marta Estrada bibliography in this volume). While rate processes seem to exhibit fast responses to physical or biological external forcing (Alcaraz 1988, Calbet et al 1996), extensive properties such as biomass and taxonomic composition are considered to follow more gradual changes through cumulative processes (i.e. growth and reproduction, Alcaraz et al 2007). Their rate of change depends on demographic parameters. Behavioural characteristics of zooplankton and advection processes strongly determine their patchy nature and the limits of what can be accepted as synoptic in terms of data representativeness and ecosystem dynamics (Steele and Henderson 1992)
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