Seasonal variations in the Indian Ocean along 110°E. IV. Macropankton and micronekton biomass

Abstract

At about 2230 hr, samples were taken between the surface and approximately 210 m with a 5-ft Isaacs-Kidd midwater trawl (IKMT) at 91 stations.

Samples were divided into four arbitrary categories-gelatinous organisms, small plankton, macroplankton, and micronekton. The last two categories were considered to be typical of the IKMT and were subdivided further. IKMT material is characterized by the dominance in weight of fishes, carids, euphausiids, sergestids, and leptocephali.

Measurements were expressed as dry weight and frequency per standard haul of 10,000 m, and as average organism size. The average dry weight of 14.8 g consisted of one-third fishes, about one-quarter crustacean macroplankton, and one-quarter small plankton.

Between 0 and 200 m meroplanktonic larvae of littoral organisms were concentrated toward the northern and southern extremities of the section. Gelatinous organisms were more abundant by weight in the south while larger crustaceans were more abundant in the extreme north. The distribution of fishes tended to be bimodal. In general, groups with maxima further to the north developed these maxima later in the season. The size of cephalopods, penaeids, and chaetognaths was usually greater in the south. Fishes, euphausiids, and carids were larger in the north. Phyllosomas, stomatopods, and leptocephali (meroplankton), and annelids and pteropods (holoplankton) were larger toward the centre.

In general the highest frequencies and largest biomasses occurred from August to November. There were periods of poverty in the first part of the year. Micronekton biomass was also high in March-April. The seasonal cycle was often clearer in the south, suggesting more marked biological seasons there, and more confused in the centre, suggesting that this region was of an intermediate nature. The seasonal distribution as a whole was bimodal. A main frequency maximum with a smaller average size of organisms was preceded by a secondary frequency maximum with a greater average size of organisms. These two maxima were related, the interval between them being of the order of 6-9 months. This suggests a succession of adults and young individuals.

There were minima in abundance characterizing whole regions and cruises, suggesting that there were large-scale environmental changes, e.g. to the south of 26º S., from mid April to the beginning of June; to the north of 12º 30'S., in January to March; and, in particular, from the north towards the centre and south, in September during the richest period of the year. Except for the end of the year, when the distribution was similar from north to south, there were alternate maxima and minima along the section.

Despite similarities in the distribution of the four major categories, their cycles of abundance were out of phase. The development of small plankton was earlier than that of macroplankton and micronekton.