Arabian Sea - Executive Summary

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General Introduction and Background

U.S. GLOBEC's goal is mechanistic understanding of population processes in the sea. The stimulus for pursuing that goal is the need to predict the consequences of global change for marine populations. Our pursuit will be fruitful if we find sites and problems offering natural experiments on population dynamics. The Arabian Sea offers two fine natural experiments of the sort U.S. GLOBEC requires:

(1) Because of the monsoonal alternation of the winds, it provides all the advantages for population studies of a seasonal climate, but provides them in a tropical setting. In addition, because the monsoons drive both coastal and separated oceanic upwelling regimes, there are a variety of contrasts among habitat types. The responses of the biota, particularly zooplankton and fish, to these seasonal and spatial variations cry out for study.

(2) Unlike many other major upwelling systems, the fish fauna of the Arabian Sea is dominated by myctophid fishes. One species, Benthosema pterotum, is arguably the largest single species population of fish in the world, with stock estimates ranging to 100 million metric tons. Possibly, their dominance derives from their ability to enter the suboxic middle depths so widely available across the Arabian Sea, and thus to obtain refuge from predation. Most exciting, this stock is annual, it turns over each year, perhaps faster. Thus, the productivity of B. pterotum is of the order of 100 million tons year-1! Many things are known about these fish, opening the way for a full modern study of their population processes. The population biology of the copepod species that are the direct prey of Arabian Sea myctophids is essentially unexamined. It is a project perfectly suited to a U.S. GLOBEC effort.

The geographic domain of particular interest lies offshore from the Arabian Peninsula and stretches toward the Indus delta. During the southwest monsoon of summer months, a jet of wind from east Africa across the open Arabian Sea to India (the Findlater jet) generates strong wind stress curl and, therefore, open-sea upwelling in a 500 km wide belt. There is massive ensuing phytoplankton production under the steady, very strong winds. During this period, the open-ocean upwelling domain borders on its Arabian side a domain of seasonal coastal upwelling, and on its Indian Ocean side a downwelling domain. The downwelling grades into the persistent oligotrophy of the tropical oceans. The northeast monsoon of winter, with winds in the opposite direction, also produces a strong production response in surface layers. There are two periods of quiet winds and reduced productivity between the monsoons.

This situation is ideal for study of adaptations of planktonic and nektonic populations to strong physical oscillation in habitat features. U.S. GLOBEC should mount an investigation in the Arabian Sea to understand its dynamics in the Present, so as to anticipate the effects of climatic change here and in other areas of the sea. To an extent the effects of climate change will be foreshadowed in the variable responses of populations to the strong interannual variations in monsoon intensity. Thus the U.S. GLOBEC investigation ideally should extend over at least several years. However, our state of knowledge of population processes in the Arabian Sea is so primitive that any initial study, particularly of zooplankton, will be of value.

The present instant offers a special opportunity for a U.S. GLOBEC Arabian Sea program because it can be done in coordination with the U.S. JGOFS, ONR/ARI, and WOCE programs that are scheduled to work in the region in 1994-96. In this document, we propose that U.S. GLOBEC work along the same sampling lines as these programs, taking full advantage of the rich suites of habitat data they will develop. U.S. GLOBEC need by no means be parasitic in these relations, JGOFS, which focuses on the primary production of organic matter and vertical fluxes from the euphotic zone, would benefit greatly from investigation of growth processes and population dynamics of mesozooplankton and mesopelagic fishes.

This document reviews the ecological problems suitable for study by U.S. GLOBEC in the Arabian Sea. Then it offers two levels of implementation plan for U.S. GLOBEC studies: (1) A modest program scaled to the funds and ship resources believed likely to be available for U.S. GLOBEC's participation (in coordination with other programs (U.S. JGOFS, ONR/ARI, WOCE) in Arabian Sea studies during 1994-1996. (2) An ambitious program designed to answer some of the exciting questions presented by the pelagic population ecology of the Arabian Sea. The latter proposes deployment of a vessel equipped for modern study of fish stocks, together with supporting studies of zooplankton populations. Both plans fit the model for U.S. GLOBEC studies: (1) target species are designated, (2) population processes of those species are the principal foci, and (3) the studies will be designed to enable prediction of the impacts of global change on those key species.

A U.S. GLOBEC Add-On to Arabian Sea Studies

The implementation committee proposes that U.S. GLOBEC sponsor a study that will take advantage of the ships and other logistics expected in the Arabian Sea region in 1994-1996. These include field programs of U.S. JGOFS, ONR/ARI, WOCE, and NOAA and imply the presence of three research vessels. U.S. GLOBEC should use some of the time still available in the schedules of those vessels. The Steering Committee should act now to obtain assignment of that time to U.S. GLOBEC.

Our interest in the Arabian Sea involves two major issues:

(1) The extraordinary temporal and spatial variation of physical forcing (monsoonal reversals, coastal and open-ocean upwelling) and its potential effects on the pelagic ecosystem, including the zooplankton and fish.

(2) The habitat features and population responses to them that results in the enormous stocks of myctophid fishes. While myctophid biology and ecology in the Arabian Sea are its most attractive features for U.S. GLOBEC work, the "add-on" plan for 1994-1996 focuses on improving our understanding of the response of the zooplankton and fish to the time and space variation in monsoonal forcing. Specifically, we propose to study mesozooplankton and fish stocks (emphasizing larvae) along the U.S. JGOFS transect from Oman out into the central and southern Arabian Sea. We recommend:

(1) a study of population structure and population dynamics of three target groups of mesozooplankton (Calanoides carinatus, Arabian Sea euphausiids, and Thalia democratica);

(2) a general planktological reconnaissance of the northern Arabian Sea, including the open-ocean and coastal upwelling regimes; and

(3) a study of the biology, ecology, distribution, and systematics of the larvae and adults of the dominant species constituting the massive myctophid stocks of the region.

A Full-Scale U.S. GLOBEC Study of Pelagic Populations

The implementation committee also recommends that the U.S. GLOBEC Steering Committee seek funding for an extensive, "full-scale" study of the dynamics of the interactions among the complex Arabian Sea physical regime, mesozooplankton stocks, and the massive stock of migratory myctophids. The principal focus should be understanding of the dynamics of the myctophid stocks together with their supporting food web and the relations of both to the special climatology of the region. An ambitious program of study can focus its seasonal comparisons along a single, onshore-offshore transect line, and the line out of Oman proposed for U.S. JGOFS work is an excellent choice. To provide a range of physical conditions and to investigate myctophids in both the coastal and oceanic upwellings, stations should be located on the shelf, over the slope, and along the line offshore. It should not be necessary to proceed beyond the limit of eutrophic conditions established by the SW monsoon beneath the Findlater jet. In addition, it will be necessary for studies to be dispersed alongshore, certainly at least from Cape Guardafi east to the southeastern limit of Pakistan. A tour of this perimeter on a seasonal basis would produce useful information on the response of the stocks to the variations of the monsoon regime.

The scale of this second-level plan remains within reasonable limits for a project funded primarily by the U.S., but it requires a dedicated ship, some specialized technology (particularly state-of-the-art sonar, a U.S. GLOBEC specialty), and a team of investigators working in the Arabian Sea for several years.