Mesozooplankton of the Arabian Sea: Patterns influenced by seasons, upwelling, and oxygen concentrations

The intensive study of the Arabian Sea during the 1990s included mesozooplankton investigations by the Netherlands, United Kingdom, Pakistan, India, Germany and the United States. Several major discoveries resulted. First, the high biomass of mesozooplankton observed during the Northeast Monsoon season is sustained by primary productivity stimulated by convective mixing and by an active microbial loop. The apparent 'paradox' of high standing stocks of mesozooplankton coinciding with low standing stocks of phytoplankton thus was resolved. Second, the Southwest Monsoon (upwelling) season supports a burst of mesozooplankton growth, much of which is exported to the interior of the Arabian Sea by strong currents and eddy activity and to depth at the end of the season when diapause causes at least one very abundant copepod to leave the epipelagic zone. Third, the oxygen minimum zone severely restricts the vertical distribution of mesozooplankton in the eastern region of the Arabian Sea. The copepod that withstands conditions in the OMZ most readily, Pleuromamma indica, has increased in abundance over the past thirty years suggesting the OMZ may have grown in size and/or intensity in that time. Fourth, the Fall Intermonsoon and Northeast Monsoon seasons are characterized everywhere by increased abundance of the cyclopoid copepod genus, Oithona. Abundances of Oithona measured in the 1990s are much higher than those of the 1930s, suggesting food web alterations over the past half-century.