Iron and grazing constraints on primary production in the central equatorial Pacific: An EqPac synthesis

Abstract

Recent studies in the central equatorial Pacific allow a comprehensive assessment of phytoplankton regulation in a high-nutrient, low-chlorophyll (HNLC) ecosystem. Elemental iron enters the euphotic zone principally via upwelling and is present at concentrations (530 PM) well below the estimated half-saturation constant (120 PM) for the large cells that bloom with iron enrichment. In addition, the meridional trend in quantum yield of photosynthesis suggests that even the dominant small phytoplankton are held below their physiological potential by iron deficiency. Grazing by microzooplankton dominates phytoplankton losses, accounting for virtually all of the measured phytoplankton production during El Nina conditions and -66% during normal upwelling conditions, with mesozooplankton grazing and lateral advection closing the balance. Nitrate uptake is strongly correlated with the pigment biomass of diatoms, which increase in relative abundance during normal upwelling conditions. Nonetheless, the f-ratio remains low (0.07-0.12) under all conditions. Iron budgets are consistent with the notions that new production is determined by the rate of new iron input to the system while total production depends on efficient iron recycling by grazers. Although the limiting substrates differ, the interactions of resource limitation and grazing in HNLC regions are conceptually similar to the generally accepted view for oligotrophic subtropical regions. In both systems, small dominant phytoplankton grow at rapid, but usually less than physiologically maximal, rates; they are cropped to low stable abundances by microzooplankton; and their sustained high rates of growth depend on the remineralized by-products of grazing. 406Landry et al.

Seasonal variations and trophic ecology of microzooplankton in the southeastern Arabian Sea

“…As a response to nutrient-enriched conditions during the PSM, the phytoplankton community existing in the OSM in the inshore locations shifts to a greater dominance of larger individuals, particularly diatoms (Landry et al, 1997). This was found to be true during the present study also since a pronounced enhancement of phytoplankton abundance was found in the inshore waters during the PSM period.…”

Primary productivity and its regulation in the Arabian Sea during 1995

“…There is, of course, no puzzle about how the pico-and nanophytoplankton are held to low concentrations despite high speci"c growth rates. E$cient grazing by protozoan zooplankton keep growth and loss iǹ balancea as de"ned by Landry et al (1997). The growth rate of micrograzers and, hence, the grazing rate were coupled to the growth rate of the small phytoplankton during all seasons in the Arabian Sea (Landry et al,1998;Brown et al, 1999;Caron and Dennett, 1999).…”

An optimization‐based model of iron—light—ammonium colimitation of nitrate uptake and phytoplankton growth

“…Large phytoplankton are also thought to be controlled at low densities by the grazing of large, generalized grazers, such as copepods, whose populations are sustained by their ability to graze on smaller phytoplankton and zooplankton size classes. The result is a system in which size classes (particularly smaller size classes) are each limited by predation (i.e., they are controlled ''top down''), whereas total phytoplankton biomass is controlled through the scarcity of iron (''bottom-up'' control), leading to ''grazer-controlled phytoplankton populations in an iron-limited ecosystem'' (Price et al 1994; see also Armstrong 1994 andLandry et al 1997).…”

“…In this scenario, resident (smallcelled) populations are kept in check by the grazing of microzooplankton (Banse 1990;Miller et al 1991;Coale et al 1996;Landry et al 1997). Larger phytoplankton species are thought to be more susceptible to iron limitation because of their smaller surface/volume ratios (Hudson and Morel 1990;Morel et al 1991;Price et al 1991;Sunda and Huntsman 1997).…”