Development, Mortality and Sinking of Copepod Eggs--A Field Investigation

We are quantifying egg abundance as a function of developmental progress, and change in depth, in order to estimate egg mortality rates of several copepod species (Calanus pacificus and Metridia pacifica). Our principal hypothesis is that copepod eggs suffer extremely high mortality. We are conducting the work in the calm waters of Dabob Bay, a fjord arm in Washington State. The planktonic eggs of both species are denser than the surrounding waters, so after being freely spawned, the eggs are negatively buoyant and sink. A priori, we estimated (from literature reports on similar species) that Calanus eggs would sink at approximately 30-40 meters per day (Peterson, 1980). We are quantifying the sinking rates in shore-based laboratory experiments under highly controlled conditions. Estimates of sinking rate could not be reliably estimated at sea because even the very limited ship motion in the calm protected waters of Dabob Bay would have introduced spurious eddies into the settling columns in which the measurements of sinking rate are made. Density of eggs is calculated using Stokes settling equation with the eggs of known size and water of known density and viscosity. Hatching time of Calanus pacificus is a function of temperature, but for temperatures typical of Dabob Bay, hatching times will be of order 34 to 48 hours. Thus, at a sinking rate, in still waters, of 35 meters per day, one might expect newly released eggs to have a depth distribution that is 48 to 70 meters SHALLOWER than the depth distribution of eggs that are ready to hatch. This is true, no matter what the spawning depth of eggs, as long as the spawning depth of eggs does not change markedly during the 48 hour period of observation. However, previous work in Dabob Bay has documented two additional features of Calanus spawning, which aid our interpretation. First, Calanus spawning is not uniform through the diel period; rather, spawning of most female Calanus occurs within a restricted time period of about 5-6 hours centered on the pre-dawn hours. We have made observations of spawning time; we find 90% of the females that release eggs in the first 24 hours release them between XX and YY. A second observation from Dabob Bay is that because of the concentration of spawning during this particular pre-dawn period, most of the eggs are spawned near the surface. Calanus pacificus and Metridia pacifica both undergo diel vertical migration in Dabob Bay, with nighttime residence near the surface, where intensive feeding occurs, and daytime residence at deep depths, where daytime light intensities are lower.

Dabob Bay offers several advantages to this study of egg mortality rate. First, compared to a more open system, like the coastal upwelling zone off Oregon, advective exchange is much lower in Dabob Bay. There are tidal effects in Dabob, but there is no strong coherent "alongshore" flow as is found off the Oregon coast in summer. This is important because we assume that we repeatedly sample a single population during our 48 hours of sampling. Previous studies by many investigators (particularly those of the Bruce Frost research group at Univ. Washington [web link or references]) have demonstrated coherent and interpretable patterns of copepod population dynamics from repeated sampling at a single site in the deep basin of Dabob Bay. Secondly, the weather in Dabob Bay is calm, with infrequent high wind events, ensuring that we can maintain time series sampling over multiple days. Moreover, strong wind events create turbulence in the surface mixed layer that complicate the interpretation of the sinking-development-mortality results using a simple sinking rate model. We are modeling both the no-wind case and the wind case using appropriate transfer functions from wind speed to mixed layer turbulence in order to evaluate the impact of wind events on our development-sinking-mortality rate results. A wind anemometer on board the research vessel R/V Barnes provides a continuous record of wind speed during the cruises, so that models can be forced using observed winds.

At sea, we sample approximately every 4 hours over a 48 hour period at a single station (see Figure) in Dabob Bay using a combination of multiple nets (5-6 depth stratified samples per sampling period) or a large pumping system (MEGAPUMP), which provides 9 discrete depth samples from a single pump deployment. We sampled during March, April, June and July 2003, and March, April, May, and June 2004.

During some months, phytoplankton biomass (predominantly diatoms) are high, and the nets after retrieval are covered with green slime. In other months, diatoms are much less abundant and the nets are relatively free of clogging. The MEGAPUMP has a flowmeter which provides early indication of clogging in real time. We attempted to sample sufficiently short periods (2-5 mins) at each depth that clogging was not a serious problem. MEGAPUMP also has a CTD to provide depth, temperature and conductivity. Net deployments included an inexpensive pressure recorder to provide indications of the maximum and minimum depth of sampling for each targeted depth. Nets were deployed vertically (and open), and retrieved at a constant rate, with closure facilitated by wire messenger which met the net on its ascent. The net was not stopped or slowed for closure. On some cruises the MEGAPUMP failed and all time series were obtained using vertical closing nets. On those cruises, the ships CTD was used to provide profiles of temperature and conductivity and therefore density. Mesh of the nets was 73 um on the nets and the MEGAPUMP. Since the eggs of both copepod species are ca. 150 um in diamter, these small mesh nets should provide quantitative estimates of egg abundance.

Also at sea, we make live collections of plankton to sort healthy adult females into individual containers which are placed in a temperature controlled incubator. Females are examined frequently (as often as every 2 hours) for the release of eggs (spawning). Numbers of eggs are recorded and females that have spawned are preserved for later measurements. Incubation containers with clutches of eggs from a single female are maintained in the incubator, until a predetermined "kill" time, at which point all of the spawned clutch are preserved. From these experiments, we obtain the egg production rate (EPR) of adult females from a random sample of the population, and also estimates of egg development time at the single incubation temperature. Attempts are made to maintain incubator temperatures within +- 2C, but sometimes this was not possible, especially in the warmer May-July cruises.

Zooplankton net samples are examined microscopically in the laboratory in Corvallis. Copepod eggs of XX species are identified and enumerated from subsamples of the full plankton sample. Random subsamples of Calanus pacificus and Metridia pacifica eggs are pipetted from the preserved sample and examined individually under both transmitted and epifluorescence microscopy; the latter after staining of nuclear material with DAPI or other nuclear stains. Eggs are categorized (e.g., staged) by the number of cells or morphology, from 1-cell, to 2-cell, 4-cell,...64 cell, early limb-bud, late limb-bud. This is done for both eggs collected from the field (of unknown age), and eggs from the shipboard incubations (of known age).

Adult female Calanus pacificus: