Abstract
Irradiance thresholds for phototactic response were determined for the marine calanoid copepod Calanus finmarchicus (Gunnerus). C. finmarchicus is one of many zooplankton species that exhibit diel vertical migration. Light is considered the main proximate cause of diel vertical migration, but irradiance sensitivity is unknown for many ecologically important zooplankton taxa, including C. finmarchicus. Here we studied phototaxis in C. finmarchicus in response to low levels of irradiance using a custom-made experimental setup under controlled laboratory conditions. The setup consisted of an aquarium with a light stimulus in one end of a raceway. A video camera and near-infrared light for illumination were applied to monitor the response to light in the horizontal plane. Low levels of irradiance were achieved using a white LED and a combination of absorptive neutral density filters and diode pulsing.
Stage V copepodites and adult females displayed negative phototaxis, and the threshold for phototactic response was 9.8 × 10− 6 μmol photons m− 2 s− 1. Adult males displayed positive phototaxis and the corresponding threshold value was 9.9 × 10− 7 μmol photons m− 2 s− 1. The results from the experiments were used to estimate the depths at which phototaxis is elicited in natural light conditions by conducting light simulations for an ocean scenario and a fjord scenario during peak spring bloom conditions. The estimated depths for irradiances that elicit a phototactic response corresponded to approximate depths of 158–186 m in the ocean scenario and 48–57 m in the fjord scenario. These depths are within the range of depth distributions of C. finmarchicus reported for ocean and fjord populations
Stage V copepodites and adult females displayed negative phototaxis, and the threshold for phototactic response was 9.8 × 10− 6 μmol photons m− 2 s− 1. Adult males displayed positive phototaxis and the corresponding threshold value was 9.9 × 10− 7 μmol photons m− 2 s− 1. The results from the experiments were used to estimate the depths at which phototaxis is elicited in natural light conditions by conducting light simulations for an ocean scenario and a fjord scenario during peak spring bloom conditions. The estimated depths for irradiances that elicit a phototactic response corresponded to approximate depths of 158–186 m in the ocean scenario and 48–57 m in the fjord scenario. These depths are within the range of depth distributions of C. finmarchicus reported for ocean and fjord populations