Oil droplet ingestion and oil fouling in the copepod Calanus finmarchicus exposed to mechanically and chemically dispersed crude oil

Abstract

The rates of ingestion of oil microdroplets and oil fouling were investigated in the zooplankton filter-feeder Calanus finmarchicus (Gunnerus, 1770) at 3 concentrations of oil dispersions ranging from 0.25 mg/L to 5.6 mg/L. To compare responses to mechanically and chemically dispersed oil, the copepods were exposed to comparable dispersions of micron-sized oil droplets made with and without the use of a chemical dispersant (similar oil droplet size range and oil concentrations) together with a constant supply of microalgae for a period of 4 d. The filtration rates as well as accumulation of oil droplets decreased with increasing exposure concentration. Thus the estimated total amount of oil associated with the copepod biomass for the 2 lowest exposures in the range 11 mL/kg to 17 mL/kg was significantly higher than the approximately 6 mL/kg found in the highest exposure. For the 2 lowest concentrations the filtration rates were significantly higher in the presence of chemical dispersant. Furthermore, a significant increase in the amount of accumulated oil in the presence of dispersant was observed in the low exposure group.

Modeling filtration of dispersed crude oil droplets by the copepod Calanus finmarchicus

“…Thus, such lethal and/or sub-lethal/narcotic responses may prevent the copepods from filtering dispersed oil droplets and lower their contribution to the overall removal of dispersed oil droplets from the water column. A concentration-dependent reduction in algae filtration rates was shown during exposure of C. finmarchicus to dispersed oil with concentrations ranging from 0.25À5 mg oil/l, however, at the lower end of that range the filtration of oil was substantial (Nordtug et al, 2015). Sustained food uptake has been shown in other copepods (Calanus helgolandicus, Pseudocalanus elongatus, Temora longicornis) during exposure to oil concentrations up to 10 ppm (Spooner and Corkett, 1979).…”

“…From experimental studies it is known that the filtration rate will diminish due to narcotic effects as the total hydrocarbon concentration (THC) increases (Nordtug et al, 2015). The effect is insignificant at low concentrations (<0.1 ppm), while at high concentrations (>5 ppm) filtration activity ceases completely.…”

“…Furthermore, exposure to oil in droplet form has been shown to decrease filtration activity in copepods (Nordtug et al, 2015;Hansen et al, 2009;Hansen et al, 2012;Cowles and Remillard, 1983). Thus, such lethal and/or sub-lethal/narcotic responses may prevent the copepods from filtering dispersed oil droplets and lower their contribution to the overall removal of dispersed oil droplets from the water column.…”

“…Adhesion of oil droplets to copepod body surface and filtering apparatus has been shown, but more importantly copepods have been shown to filter and ingest oil droplets during exposure to dispersed oil (Nordtug et al, 2015;Gyllenberg, 1981;Hansen et al, 2009;Hansen et al, 2012;Conover, 1971;Spooner and Corkett, 1979). The size distribution of oil droplets from a spill incident may in some cases have significant overlap with the corresponding distribution of natural copepod food particles, and oil droplets will then be ingested as food, as found experimentally by Hebert and Poulet (1980) (Hebert and Poulet, 1980), as well as others (Conover, 1971;Lee et al, 2012;Almeda et al, 2014aAlmeda et al, , 2014b.…”

“…The filtration rate for C. finmarchicus has been estimated at 40e98 ml/(day  ind. ), representing an ability to filter a water volume of 100 000 times of its body volume per day (Nordtug et al, 2015;Meyer et al, 2002). Their abundance in the pelagic environment is high (annual production of 300 million tons) with of up to 3000 copepods per m 3 (Helle, 2000) in the upper pelagic zone (¡50 m depth (Unstad and Tande, 1991)) where they filter algae.…”