Production Biology of Phytoplankton

In-situ measurements of phytoplankton chlorophyll in the Arabian Sea were taken largely along temporally and spatially unevenly distributed sections, which were scarce especially prior to the operation of NASA's Coastal Zone Color Scanner (CZCS). Herein, the CZCS pigment observations between late 1978 and mid-1986 north of 10N, including the outer Gulf of Oman, are depicted for 14 subregions beyond the continental shelves as daily means, often only five days apart. To eliminate bias from electronic overshoot, the data were reprocessed with a more restricted cloud screen than that used for NASA's Global Data Set. The pattern, derived from the older in-situ observations, of one period with elevated chlorophyll almost everywhere during the southwest monsoon (SWM) and one additional late-winter bloom in the north, is confirmed. The differing nitrate : silicate ratios in freshly entrained water in the central and northern Arabian Sea seem to lead to different succession and perhaps to differing vertical fluxes, and during winter favor blooms only in the north. The spatial pigment pattern in the outer Gulf of Oman is not an extension of that of the northwestern Arabian Sea. The physical seasonal forcing explains much of the timing for changes in pigment concentrations, but not the levels maintained over long periods. From the CZCS observations it is unclear whether the period of high phytoplankton productivity expected during the SWM in the open Arabian Sea lasts for about two or four months. During this entire season, chlorophyll values in the upper layers rarely exceed 1 2 mg m outside the zone influenced by the Arabian upwelling. Near 15N, however, fluxes into sediment traps at 3 km depth indicate an onset of high primary production very soon after the arrival of the SWM and suggest a long open-sea period of high production. The temporal, partial disconnect during the SWM between pigment changes in the upper part of the euphotic zone and of fluxes into the traps is disconcerting. For future modeling of plankton production in the open Arabian Sea, the use of two size classes of phytoplankton is recommended. The utility of satellite-derived pigment concentrations (as opposed to temporal changes of pigment) for testing such models is questioned. IMPACT 1. The significance of the two published works named under RESULTS and the earlier regional studies in the Subantarctic parts of the Southern Ocean initiated with NASA support but written up with ONR support, are as follows: [a] The regions were objects of JGOFS field work; our papers will help to set these short-term expeditions (usually during one year) into the long-term context. [b] For a few more years, the CZCS time series will be the longest pigment series available for most of the oceans. Now, for a few offshore regions outside of the North Atlantic, the CZCS archive has been mined for detailed seasonal descriptions, with weekly or even shorter resolution, and inter-annual differences also have been studied. [c] For these regions, almost monographic evaluations of spatial and temporal changes have been attempted on the mesoscale and larger scales of phytoplankton chlorophyll and the relevant hydrographic, nutrient, and zooplankton data. Some results of the JGOFS field work have been incorporated in these syntheses. It was stressed that the seasonal change of phytoplankton abundance (as chlorophyll) cannot be understood from phytoplankton physiology, i.e., light, mixing and nutrients. 2. The translations from the Russian will be of value by themselves and will provide access to a large body of literature largely unknown among the English-only reading colleagues. The two books by Sazhina have no counterparts in the western literature, and the two regionally oriented books concerned with the Arabian Sea contribute not only historical material, centered on the period 19801990, but also will allow three-dimensional modeling of processes. This is not reasonably done when based on the two-dimensional sections run by the JGOFS (and most earlier) expeditions. TRANSITIONS As stated under Impact, the new regional and temporal results derived from the CZCS and the accompanying reviews of in-situ knowledge should help the community, especially the JGOFSfocused colleagues, in their integrations and syntheses. Only time will tell whether this will be so. RELATED PROJECTS Collaboration with colleagues at India's National Institute of Oceanography in Goa and the Central Marine Fisheries Research Institute in Cochin continues. The focus is on hydrography including regional and temporal distribution of oxygen, as well as nitrogen metabolism on and off the shelf off the west coast of India. PUBLICATIONS Banse, K. and D.C. English (1999) Comparing phytoplankton seasonality in the western and western subarctic Pacific and the Bering Sea. Prog. Oceanogr. 43: 235-288. Banse, K. and D.C. English (In Press) Geographic Differences in Seasonality of CZCS-Derived Phytoplankton Pigment in the Arabian Sea. Deep-Sea Res. II (70 mss. pp.).