Development, Mortality and Sinking of Copepod Eggs, a Field Investigation

Oregon State University, Corvallis OR

Investigators

Abstract

Eggs of free-spawning copepods are reported to have extremely high mortality rates. Typical estimates of losses before hatching exceed 90%. Most planktonic eggs of near-surface spawning plankton sink, rather than rise. Thus, it is expected that embryo age will, on average, increase downward in the water column as numbers sharply decrease. This study will quantify egg abundance as a function of developmental progress, thus producing estimates of egg mortality rates, and to examine the vertical distribution of egg development stage. Sampling will be carried out with MEGAPUMP, an overside, 2 m 3 min -1 pumping system that has nine sequential filters mounted in a carousel and an onboard CTD system. Lowering MEGAPUMP through the water column with the pump off will produce temperature and salinity profiles. Pumping on retrieval will produce a 9-level, stratified abundance profile of 50 mm zooplankton, including the eggs of all but the smallest calanoid copepods. The study will be conducted in the calm waters of Dabob Bay, a fjord arm in Washington State, deploying MEGAPUMP from the UNOLS vessel R/V BARNES. Four cruises will be conducted in each of two years, 2003 and 2004. Two cruises in the first year will be in cooperation with a group at the Univ. of Washington headed by Dr. Bruce Frost. They are studying the mortality imposed on eggs by a diatom diet. Proposed OSU work will complement the UW project, since they do not propose water column sampling of eggs. UW sampling in Dabob Bay is scheduled weekly for February through April, intending to sample before, during and after the reliable diatom bloom that occurs in this fjord in March. The two programs are compatible aboard. Two OSU spring cruises are proposed for 2004, and two summer cruises are proposed in each of 2003 and 2004. In summer, female copepods will still be abundant and reproducing, but the water column will not have abundant diatoms. Target species for both OSU and UW programs will be Calanus pacificus and Metridia pacifica, the dominant free-spawning copepods in Dabob Bay. Usual abundances and fecundities of both species are high enough to produce large numbers of eggs from samples of only a few cubic meters, readily done with MEGAPUMP. These copepods have large, specifically recognizable eggs. Egg development will be classified morphologically and by DNA content, which is a function of cell number. Cell number in these, and at least some other copepods, is around 2000 at hatching. Thus, DNA should increase through 10 doublings and provide a finely divided development stage series. Mortality rates will be calculated from estimates of egg abundance in each stage. Broader Implications: Copepods are the dominant zooplankters in both oceanic and coastal waters. They are a main pipeline from primary production to fish suitable for harvest, often perhaps as the first consumers at the large-animal side of the microbial loop. It is likely that a major part of the transfer in that pipeline is via copepod eggs. For example, copepod eggs and early nauplii are the main food for larvae of North Atlantic cod. Spawning female copepods are not just stocking future generations; they are feeding much of the food web above them. Well quantified egg mortality rates will be data essential to accurate modeling of copepod population dynamics, rates for which few estimates exist. A study in Dabob Bay will be a prelude to studies in more complex coastal and oceanic habitats. The work will contribute to the overall goals of planktology at the present day, as exemplified in the U.S. and International GLOBEC programs. Training of a student by the project will contribute to the future of planktology.