English abstract
Zooplankton are considered an important food source to small pelagic fish. An analysis of the spatial-temporal variability of the zooplankton community inhabiting the Omani shelf waters was basically skipped by numerous International expeditions devoted to the Arabian Sea.
However, zooplankton sampling was part of five seasonal surveys carried out along the shelf, from the Ras al Hadd region to the Yemen border. The collected samples have not been processed before. In this regard, the goal was to analyze the spatial distribution and seasonal changes in the abundance of characteristic species, as well as the total zooplankton biomass.
The biomass was determined for 216 samples 20 of which were processed to the level of species. Zooplankton abundance and biomass were dominated by copepods. Spatial distribution of zooplankton biomass was highly heterogeneous. Local maxima with up to 2000 mg m-3 were observed. The mean values were 689, 723, and 543, mg m3 for the North-east Monsoon,
Spring Inter-monsoon, and South-west Monsoon seasons, respectively. No statistical differences of zooplankton biomass were found between these seasons.
An absence seasonality on the level of total zooplankton biomass was accompanied by relatively stable species composition of the dominant (by abundance) copepods; 40 species were found during the North-east Monsoon, 38 species during the Spring Inter-monsoon, and 34 species were observed in the South-west Monsoon. Of those, 11 species (about 30 %) were
present throughout the year. Seasonally invariant zooplankton biomass was accompanied by the seasonality observed on the level of species abundance and community size structure. The most striking feature was the massive appearance of the copepod Calanoides carinatus (KrĂyer, 1849), in upper layers, to which the population migrated from the depth of about
400m, during the South-west Monsoon. In terms of spatial distribution, the population, ascending to upper layers during an
ontogenetic migration, occupied the entire Omani shelf area from Salalah to Ras al Hadd regions.
The seasonal variation of the size structure explains in part the mechanism of seasonally invariant zooplankton biomass. The contribution of large-sized organisms is relatively high during the North-east Monsoon and gets less pronounced through the Spring Inter-monsoon to the South-west Monsoon, in which the small-sized zooplankton dominate. The weight of
zooplankton organisms is a power function of their size. This means that less abundant large- sized organisms could be replaced by abundant small-sized organisms which could compensate the decrease of the total zooplankton biomass.
The trophic structure of copepod community demonstrated seasonal changes as well. In particular, carnivores dominated the North-east Monsoon and Spring Inter-monsoon seasons while herbivores were dominant during the South-west Monsoon.
The Principal Component Analysis of 16 environment variables implied the sea surface temperature, wind speed, number of cyclonic eddies, euphotic depth and the biomass of small pelagic fishes to be the dominant in stabilizing the zooplankton biomass over seasons (the so called "Zooplankton Paradox").
