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.

The bottom waters of the mesohaline portion of Chesapeake Bay become depleted in oxygen in summer. We found that copepods and nauplii were in low abundance or absent from bottom waters when oxygen concentrations were < 1 mg 0, litcr-I. In contrast, when oxygen concentrations were higher in bottom waters in spring or summer due to episodic mixing events, the highest copepod abundances were often found in bottom waters. Laboratory experiments confirmed that oxygen concentrations < 1 mg 0, liter-1 resulted in reduced survival of the copepods Acartia tonsa and Oithona colcarva and inhibited the hatching of A. tonsa eggs. The decrease in Chesapeake Bay copepods in May-June parallels the decline of oxygen in bottom waters. Our field and laboratory data suggest that this dcclinc in copepods could result from reduced recruitment as a consequence of egg mortality in the low-oxygen bottom waters. In summer this source of mortality would be reduced because warmer water temperatures would allow the eggs to hatch in the upper water column above the low-oxygen bottom waters.

Outbreaks of jellyfish are reported worldwide, yet the environmental factors that control the sizes of jellyfish populations are not well understood. The scyphomedusan Chrysaora quinquecirrha occurs in the mesohaline portion of Chesapeake Bay each summer. Population sizes of the medusae show dramatic annual variations that are correlated with salinity and temperature. We measured the total numbers of ephyrae and polyps produced by benthic polyps of C. quinquecirrha in laboratory experiments lasting 42 d, and found that temperature (15. 20, 25°C) was not a statistically significant factor at low salinities (5 to 20P:m); however, ephyra production increased significantly with increasing temperature at high salinities (20 to 35%). Conversely, each 5°C decrease in temperature delayed strobilation (ephyra production) by about 1 wk. Salinity significantly affected the numbers of ephyrae and polyps produced in all experiments. Ephyra and polyp production was lower at both low (

Gelatinous zooplankton are conspicuous p'redators during summer in Chcsapeakc Bay. Inverse correlations of the abundances of gelatinous predators and their zooplankton prey have led to speculation that these predators may control their prey populations. We measured predation on copepods by hydromedusae, scyphomedusae, and ctcnophorcs in the mesohalinc region of the bay in May-October 1987 and. WC simultaneously asscsscd the possibility of food limitation in Acartia tonsa by measuring its phyto-and microzooplankton food resources and rates of adult female egg production. WC conclude that A. tonsa populations arc not limited by gelatinous zooplankton predation and rarely by food. Other predators (c.g. fish, adult copepods) and environmental limitations (e.g. temperature, hypoxic bottom waters) probably affect A. tonsa abundances in this region.