Changes in respiration and excretion rates of boreal zooplankton after capture (Saanich Inlet, British Columbia, Canada)

The herbivorous copepods, Calanus plumchrus and Paracalanus parvus, the herbivorous euphausiid, Euphausia pacifica, the carnivorous amphipod, Parathemisto pacifica and the carnivorous ctenophore, Pleurobrachia pileus used in this experiment were collected from Saanich Inlet, British Columbia, Canada, between May and August 1975.

All animals, except for Paracalanus parvus, which were used immediately, were left overnight in unfiltered sea water collected from the same site as the zooplankton. Zooplankton were then maintained in 300 to 500 ml beakers containing well aerated filtered seawater in a constant temperature room at 10°C ± 1°C.

Two different feeding regimes were used for the experiment, with one group of each species fed excess food and the other starved by placing the zooplankton in filtered sea water only. Calanus plumchrus, Paracalanus parvus and Euphausia pacifica were fed a diet of cultured Chaetoceros spp., originally collected from the same site as the zooplankton. Parathemisto pacifica were fed chopped fresh Euphausia pacifica and Pleurobrachia pileus were fed freshly collected Calanus plumchrus.

Prior to starting the experiment, a subsample was taken from each species, except for the ctenophore, Pleurobrachia pileus, to establish the mean dry body weight of the experimental animals. At the end of the experiment, the mean dry body weight of the survivors was also determined. Dry body weight was measured by placing specimens in a desiccator over silica-gel at approximately 20°C for 1 week. For Pleurobrachia pileus, the equatorial diameter, measured by placing each specimen with a small amount of sea water in a shallow dish under the dissecting microscope was used as a measure of body size.

Respiration rate was measured with an oxygen electrode (YSI oxygen monitor system). Zooplankton were incubated in a 6.3 ml cell with filtered water for 20 to 30 minutes. Rates of ammonia and inorganic phosphate excretion were measured after removing the zooplankters from the cell, rinsing with filtered sea water and incubating in 30 to 40 ml of filtered sea water in a small beaker fitted with a glass lid for 2 to 4 hours. Ammonia and inorganic phosphate were determined by the phenol-hypochlorite and molybdate methods, respectively. Zooplankters, except for Paracalanus parvus, which was too small to handle repeatedly without damage, were then returned to beakers, with or without food, for subsequent measurements. Depending on the species, zooplankters were maintained between 3 and 49 days, with between 3 and 10 respiration and excretion measurements taken over the maintenance period.

On day 34 of the experiment, the body surface of starved Calanus plumchrus specimens were observed to be covered with epiphytic pennate diatoms, which were later identified as Navicula spp. Licmophora sp and Synedra sp.. By day 49 both starved and fed Calanus plumchrus were covered with these diatoms.