THE PROBLEM OF THE ISLAND MASS EFFECT ON CHLOROPHYLL AND ZOOPLANKTON STANDING CROPS AROUND MARE (LOYALTY ISLANDS) AND NEW CALEDONIA

The island effect on plankton biomass is tested at two different sites: Mare, a coral island directly facing the prevailing surface currents with no lagoon and no land drainage; the second site to the southwest of New Caledonia has a wide lagoon and important run-Offs. Since the sole coastal chlorophyll “a” increase (<20%) is observed at the latter site and no correlation is found with vertical mixing or the thermal structure, we conclude that such an island effect is mainly due to transports from land and lagoon. Secondly, there is no relation between integrated chlorophyll “a” and zooplankton dry weights, like in most neritic ecosystems, because of the short-term variability (scale of days) of the meteorological and hydrological parameters. Finally, significant mid-term (scale of months) biomass variations are observed in this oligotrophic environment. The “island mass effect” on phytoplankton and zooplankton biomasses and production is rather well documented, although the causative mechanisms are variable and not always obvious: eddies (Emery, 1972), upwelling in the wake of the island (Moore, 1949), increase of the amplitude of internal waves (Sander, 1973; 198 11, vertical mixing in island channels (Gilmartin and Revelante, 1974), andinteractions with the benthos and run-off from land (Doty and Oguri, 1956). The general result of these studies (see discussion^' for references), is a coastal enrichment of.phytoplankton and for most of them, of zooplankton, with a possible positive gradient from off- to inshore. The search for such an effect is particularly important in oligotrophic areas, such as the southwestern Pacific Ocean, and is the aim of the PROCAL (PROduction CALedonienne) program of ORSTOM oceanographic center in Noumea. Two sites, with different enrichment mechanisms, were chosen. The first, Mare Island, which is the southernmost Loyalty island (Fig. l), may be influenced mainly by rather simple modifications of surface currents, without any disturbance by other islands since it receives the southeasterly trade winds, directly. Moreover, Mare is a coral island approximately 30 km wide, that was lifted above the sea level by tectonics (maximum altitude: 138 m) and has almost no land drainage. Finally, it has no continental shelf and, therefore, no lagoon and reduced influence from the benthos. The second site is off southwestern New Caledonia (Uitoe transect, Fig. 1) and represents a different case: besides the hydrological modifications that are expected around New Caledonia, land-drainage and exchanges with the wide lagoon (10 n.m. at Uitoe) are other important possible effects on plankton production of the near open-ocean. In order to test the possible increase in phyto- and zooplankton standing crops, a series of transects were made around Mare and off sw New Caledonia, each transect consisting of three stations, at 2, 10 and 30 nautical miles from the coast, the first station being no closer to the coast for safety reasons. Each cruise around Mare was in January, April, September and November 1983, with R.V. CORIOLIS, and consisted of 6 transects (Fig. 1). Each cruise included a 2-day survey of the surface currents and one or two severalday-long stations for the study of short-term variability of hydrological and planktonic parameters -vertical distribution and production assessments. The second