Relationship of predation potential to mortality of Calanus finmarchicus on Georges Bank, northwest Atlantic

Based on 29 cruises on Georges Bank between January and June, conducted as part of the U.S. Global Ocean Ecosystem Dynamics program, we describe seasonal and spatial variability of early life history mortality for the planktonic copepod Calanus finmarchicus and relate mortality to an index of predation potential from a suite of suspension-feeding predators. Emphasis is placed on the earliest life history phase wherein peak mortality occurs. Instantaneous mortality rates for a combined egg-through-nauplius-3 stage show a recurrent seasonal pattern of a modest elevation in January followed by a seasonal peak in May of all 5 study yr. Spatial differences exist in mortality rates, with a higher probability of mortality on the southern flank in winter and a pronounced seasonal maximum on the bank crest in May. Three hypotheses associated, respectively, with egg hatching success, advection, and invertebrate predation were evaluated to account for the seasonal and spatial mortality patterns. Variations in egg-hatching success are inconsistent with the observed seasonal patterns of loss. Off-bank advection, particularly on the southern flank, appears to be the dominant loss term in January. Apart from this winter period, egg mortality is associated with predation potential. Comparison of an index of daily rates of egg loss with an index of consumption rates of eggs by suspended hydroids, hydromedusae, and five species of planktonic copepods show general agreement between mortality and predation in both seasonal variation and spatial patterns. Of the predator taxa, late copepodid and adult C. finmarchicus, supplemented by planktonic hydroids, appear to have the largest quantitative predation effect. Zooplankton populations in temperate latitudes characteristically experience strong seasonal forcing of water column stability, temperature, phytoplankton production, horizontal transport, and the influence of predators. Efforts to understand the mechanisms controlling zooplankton population dynamics have traditionally focused on effects of the first three of these processes. These seasonal drivers are usually emphasized in numerical simulations that seek to reproduce zooplankton population dynamics. Although alternative perspectives have been presented, focusing on the seasonality of predation (e.g., Ohman 1983; Davis 1984a,b; Purcell and Decker 2005) and advective losses (e.g., Lewis et al. 2001), the role of the loss term in governing the temporal and spatial variability of suspension-feeding marine zooplankton remains an understudied aspect of zooplankton ecology. An impediment to addressing mortality and other losses in the marine pelagic environment, apart from specialized environments such as lagoons (Landry 1978), embayments (Uye and Liang 1998), and fjords (Eiane et al. 2002), has been the difficulty of sampling the same planktonic population over time. A basic concern is that the residence time (t) of the water and organisms in the study region 1 Present address: Monterey Bay Aquarium Research Institute, Moss Landing, California 95039-9644. Acknowledgments We thank Stéphane Plourde, Pierre Joly, and Lucienne Chenard for their assistance with egg production and hatching success measurements, Maria Casas for her help throughout this research, and the reviewers for their comments. J.R. acknowledges Fisheries and Oceans, Canada, for its support of collection of data for this study. This research was supported by National Science Foundation and National Oceanic and Atmospheric Administration grants awarded to E.D., J.R., and M.O. This is contribution 580 from the U.S. Global Ocean Ecosystem Dynamics (GLOBEC) Northwest Atlantic/Georges Bank program. Limnol. Oceanogr., 53(4), 2008, 1643–1655 E 2008, by the American Society of Limnology and Oceanography, Inc.