We present the first comparative study of the stage-specific patterns of mortality of Calanus and Pseudocalanus, two widely distributed genera that are representative of a relatively large-bodied, broadcast spawning calanoid copepod and a relatively small-bodied, egg-brooding calanoid. The study site is Georges Bank, a continental shelf locality in the Northwestern Atlantic with retentive circulation that renders it suitable for studies of population dynamics. Based on extensive mortality estimates from 30 cruises, we find that co-occurring Calanus finmarchicus and Pseudocalanus spp. have markedly different patterns of stage-specific mortality, the former bimodal and the latter relatively uniform with respect to developmental stage. Neither taxon exhibits a monotonic decline in mortality with developmental stage, nor are rates of mortality predictable in a useful manner by copepod body size or by ambient temperature. Young stages of the broadcast-spawning C. finmarchicus show conditional density-dependence of mortality rates, i.e. mortality rates are independent of population density when adult females are low in abundance but positively related to population density at high female abundances. This density-dependence, which is probably attributable to egg cannibalism, introduces a quadratic mortality term into population dynamic models. The egg-brooding Pseudocalanus spp., in contrast, show no evidence of density-dependent mortality. The two taxa illustrate a life history trade-off: the broadcast-spawning Calanus exhibits birth rates that are greatly elevated with respect to those of Pseudocalanus, but there is a compensatory cost in very low survivorship of the freely spawned eggs. Both the high fecundity, high mortality life history of Calanus and the low fecundity, low mortality life history of Pseudocalanus appear to have approximately equal fitness in this study site.

It is currently unclear whether observed pelagic ecosystem responses to ocean warming, such as a mid-1970s change in the eastern North Pacific, depart from typical ocean variability. We report variations in planktonic foraminifera from varved sediments off southern California spanning the past 1400 years. Increasing abundances of tropical/subtropical species throughout the 20th century reflect a warming trend superimposed on decadal-scale fluctuations. Decreasing abundances of temperate/subpolar species in the late 20th century indicate a deep, penetrative warming not observed in previous centuries. These results imply that 20th-century warming, apparently anthropogenic, has already affected lower trophic levels of the California Current.

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. © 2008, by the American Society of Limnology and Oceanography, Inc.