Appendix B. Zooplankton Working Group Report

Chair: Karl Banse

Rapporteur: Hal Batchelder

Participants: Ann Bucklin, Dave Checkley, Tim Cowles, Bill Peterson, Karen Wishner

Introduction

The western Arabian Sea offers a unique opportunity to study pelagic ecosystems subject to very strong seasonality in physical forcing by the reversing monsoons, with considerable interannual fluctuation correlated with ENSO signals, and a known paleontological record of past climate and climatic effects. The geographic domain of particular interest lies offshore from the Arabian Peninsula and stretches toward the Indus delta. During the summer, southwest monsoon, the Findlater (East African) jet leads to open-sea upwelling and ensuing phytoplankton production under rather steady, very strong winds. During this period, the approximately 500km wide open-ocean upwelling domain is, toward Arabia, adjoined/bordered by a coastal domain with strong seasonal upwelling, and on the seaward side by a central Arabian Sea downwelling domain, which intergrades into the persistently oligotrophic domain of the eastern and southern Arabian Sea. North of 20 deg N lies a fourth domain which has two phytoplankton blooms per year, one wind-driven and concurrent with the blooms in the upwelling domains, and another caused by nutrient injection during winter surface cooling and convective overturn.

We expect that the major goals of U.S. GLOBEC, to study biological processes (e.g. growth, production, mortality) of dominant species and the causes of fluctuations of their abundance in relation to the physical regime, can most profitably and uniquely be investigated in such regimes of physical contrast but which are, at least in part, inhabited by the same species of zooplankton and mesopelagic fish. We believe that adaptations of such planktonic and nektonic populations to strongly physically forced habitats should be investigated to understand the Present, as well as to anticipate the effects of future climate change here and in other areas of the sea where climate change might lead to conditions similar to those found in the four domains of the Arabian Sea. Such a study would best be done in coordination with other programs (U.S. JGOFS, ONR/ARI, WOCE) working in the same area, or even at the same stations. In fact, we suggest that U.S. JGOFS, which focusses on the primary production of organic matter and vertical fluxes from the euphotic zone, but does not consider in any substantive way the higher trophic levels, would benefit greatly from a U.S. GLOBEC oriented investigation of growth processes and population dynamics of mesozooplankton and mesopelagic fishes.

The zooplankton working group developed the following primary questions: How do the structure and dynamics of ecosystems in coastal and open-sea upwelling domains in the Arabian Sea differ? To what extent are the domains biologically distinct, i.e., containing genetically distinct populations among the abundant species occurring in two or more domains? What are the determinants of community composition, e.g. physiological adaptation, fecundity, growth, mortality, and genetic isolation? Note that these questions are not restricted to zooplankton populations; they are valid questions for mesopelagic fish populations as well. That was intentional. General and specific scientific questions which were discussed by the working group are detailed below and posed as questions rather than as hypotheses.

General Themes and Specific Scientific Questions

Discussion focused on two general themes. The first focused exclusively on the plankton dynamics and structure of the open-ocean upwelling domain, the second on a comparison of plankton structure and dynamics among the various physical domains of the north Arabian Sea.

General Theme: To describe and understand the higher trophic level structure and dynamics of the physically-unique, open-ocean upwelling system in the Arabian Sea.

Specific Questions:

Are there large suspension-feeding mesozooplankton in the central north Arabian Sea that are capable of taking advantage of the strongly seasonal pulses of food? If so, what are they? What are their vital (development, growth, reproduction, mortality) rates? If such grazers, typical of coastal upwelling systems, are not a dominant feature in the central Arabian upwelling region, why not? And by what are they replaced?
What are the trophodynamic links between the high and sustained primary productivity during the several months of the SW monsoon and the enormous populations and biomass of midwater fish, especially myctophids, in the Arabian Sea?
What is the fate of the enhanced primary production associated with the SW monsoon in the central Arabian Sea? Is it grazed? Does it ultimately enhance fish production in the central Arabian Sea? Does it end up on the bottom to enhance benthic production? This would appear to be a principal question of U.S. JGOFS studies in the Arabian Sea.
To what extent does the oxygen minimum zone (at ca. 150-1500 meters) in the central and eastern Arabian Sea restrict the vertical distribution and vertical migration of zooplankton? Is the oxygen minimum in some way responsible for the apparently low species diversity of the mesopelagic animal groups (fish, euphausiids, copepods) in the northern Arabian Sea?
What is the response of the zooplankton (and higher trophic levels) populations to the strongly seasonal physical forcing? How does the strong seasonality affect species composition and abundance and trophodynamic linkages among the higher trophic levels? How do the population dynamics (vital rates) of the zooplankton vary among the SW monsoon, NE monsoon, and transitional periods?

General Theme: How do the upper trophic level structure and dynamics in the open-ocean upwelling domain compare to those in adjacent physical domains of the Arabian Sea: the Arabian Peninsula coastal upwelling domain, the permanently oligotrophic region of the southeastern Arabian Sea, and the Arabian Sea north of 20 deg N.

Are there substantial differences in the pelagic biota (net zooplankton and fish fauna) species composition and food web dynamics between the coastal Arabian Peninsula (Oman) upwelling region and the central Arabian Sea upwelling region?
Unlike the central Arabian Sea, the area north of 20 deg N exhibits two phytoplankton blooms, one during each of the SW and NE monsoons. The physical dynamics responsible for the two blooms differs markedly. The primary productivity during the SW monsoon is driven by wind-induced upwelling of deeper waters, whereas the NE monsoon bloom occurs as a response to convective overturn due to winter cooling. How do the plankton structure and dynamics of the two regions differ?
How does the zooplankton composition and dynamics in oceanic regions of the eastern Arabian Sea, which are permanently oligotrophic, differ from regions where upwelling occurs?

What are the physical and biological linkages between the open-ocean upwelling domain of the Arabian Sea and the adjacent physical domains? The gradient from the coastal upwelling domain off Arabia (Oman) to the broadscale Findlater Jet driven offshore upwelling domain to the permanently oligotrophic domain in the eastern Arabian Sea is the region of interest to U.S. JGOFS. Detailed process studies and intercomparisons of the dynamics and structure of the upper trophic levels in these different domains by U.S. GLOBEC should be of immense value to, and nicely complement, the studies planned by U.S. JGOFS.

Logistics Recommendations

Examine the extant literature on zooplankton species composition, community structure and dynamics in the Arabian Sea. Unlike many other regions of the world's oceans, the information on the zooplankton of the Indian Ocean, including the Arabian Sea, is sparse. The data that have been worked up are scattered, often in foreign languages. It was the belief of the zooplankton working group that one such resource is the substantial information on the abundance and distribution of zooplankton and mesopelagic fishes in the Arabian Sea generated by Soviet research cruises. We recommend that one or several U.S. investigators collaborate with Russian oceanographers in the Sevastopol and Moscow institutes, with the goal of writing and publishing reviews (in English) of the accumulated results of Russian (formerly Soviet Union) investigations of the zooplankton and fish ecology of the Arabian Sea and Indian Ocean generally. These reviews should also include data from regional countries near the Arabian Sea, eg. India and Pakistan, to the extent that such inclusion is possible.
As an aid in defining the distribution of mesopelagic fishes and zooplankton biomass, acoustic data on scattering layers (DSL) in the open Arabian Sea should be collated and examined.
The working group felt that initial survey cruises, using Seasoar (or similar) techniques to define the biological, optical, acoustic, and physical properties of the various oceanographic domains, and to define the scales of variability of these parameters would be valuable as part of U.S. GLOBEC studies of upper trophic level structure and dynamics. Such survey cruises should be conducted prior to each major oceanographic process cruise sponsored by U.S. GLOBEC.
A consensus was established that to "piggyback" a large U.S. GLOBEC scientific effort on U.S. JGOFS cruises, or to be restricted to ship time when other programs were idle, was not the preferred method for conducting a viable U.S. GLOBEC study of the upper-trophic levels of the Arabian Sea. Rather, the group felt that U.S. GLOBEC should request a moderate-to-large, modern, well equipped UNOLS vessel that would be dedicated to U.S. GLOBEC investigations. Moreover, it was felt that with two UNOLS vessels (one for U.S. GLOBEC, one for U.S. JGOFS) in the Arabian Sea during the proposed study period, there would be enhanced opportunity for exchange of scientists between the two programs, as well as potentially better seasonal coverage of the dynamic oceanographic conditions of the Arabian Sea.

Survey Mode and Measurement Recommendations

The principal goal of the survey operations is to define the gradient in property fields in the coastal upwelling domain, the offshore upwelling domain, and the oligotrophic domain of the northern Arabian Sea. Surveying should be done using an underway, towed undulating body instrumented with sensors for biology, physics, chemistry, optics, and bioluminescence. Multiple frequency acoustics should be used on towed bodies, or vessel mounted to provide detailed information on the distribution of biomass and size frequency of zooplankton. Vessels conducting the oceanographic surveys should be equipped with acoustic doppler current profilers for underway measurement and two dimensional mapping of current velocities and acoustic backscatter (an analog for zooplankton/micronekton biomass). Survey vessels should be equipped with modern meteorological sensing packages, capable of continuous, frequent observations and should be equipped with serial ASCII interface loop (SAIL) systems, and shipboard instruments (automatic size counters, fluorometers, thermistors, etc.) for routine underway measurement of near-surface optical, biological, bioluminescent, and physical properties.

In addition to underway measurements, we recommend that short-stay stations be done. The purpose of the short-stay survey station measurements is to provide additional ancillary data, or extended depth range data, that cannot be obtained directly using towed undulating sensors. An example of the former is the use of nets and or pumps to provide samples for the evaluation of zooplankton biomass and species composition and for collection of specimens for molecular/biochemical studies. An example of the latter, is hydrographic profiles to greater depths, esp. within the oxygen minimum zone, than can be sampled by towed packages (200-300m). In addition, the survey station measurements will provide the data needed for sea-truthing remote sensing devices (fluorometers, acoustics, etc.). The working group suggested that possibly 1-2 of these sea-truthing stations per physical domain (coastal upwelling, offshore upwelling, oligotrophic) might be sufficient. However, more frequent sampling might be required in regions having large gradients.

Extended Station Mode and Measurement Recommendations

The primary goal of the extended station measurements is to obtain an understanding of the population dynamics and structure of the upper trophic levels (zooplankton and fish) of the Arabian Sea. Toward that end, the zooplankton working group strongly recommends that U.S. GLOBEC consider extended stays at fewer stations, one or more in each of the three different physical domains. The structure (abundance, distribution, species composition, and genetics) of zooplankton populations should be measured using direct (nets, pumps) and indirect (acoustical and optical) means. Sampling should be vertically stratified to reflect the strong vertical gradients (especially oxygen concentration) that may control (or restrict) animal distribution and determine physiological condition. Moreover, measurements should be done throughout the diel cycle. Zooplankton dynamics (rates) that should be studied include trophic interactions, population dynamics, and individual processes. Trophic interactions (e.g. consumption, mortality) should be investigated in each of the physical domains as a function of depth and the diel cycle. Population dynamics (e.g. vital rates of birth, development, and mortality rates) should be examined in the dominant zooplankton species, whatever they may be. Individual processes (e.g. feeding, metabolism, growth, defecation, and behavior) should be investigated in the dominant plankton. The dynamics of the zooplankton should be investigated by a combination of shipboard experiments, in situ observations, and if possible in situ experiments.