Zooplankton population dynamics:: measuring in situ growth and mortality rates using an Optical Plankton Counter

An application to assess in situ population dynamics rates is introduced based on the measurements of size-structured zooplankton population. This method differs from those based on species and stages by taking advantage of allometrically-scaled rates, automated sampling devices and net tows. For the first test of this method, we conducted 3 surveys in a nearly enclosed sub-Arctic fjord in northern Norway, from early May to mid June 1998, the period of the spring bloom. Bio-volume spectra in the size range between 10(-1) and 10(3) mm(3) individual body volume, from small mesozooplankton to euphausiids, were derived from the Optical Plankton Counter measurements, From 3 bio-volume spectra measured on Days 1, 21, and 35, we calculated in situ individual growth and mortality rates using the theory based on biomass spectra. The net samples corroborate the basic fact that Calan us finmarchicus and euphausiids absolutely dominate in 2 different size ranges. In this spring bloom period, the first generation of Calan us finmarchicus at copepodite stage CIV dominated the size range between 0.3 and 0.58 mm(3), and had a growth rate within 0.02 and 0.17 d(-1). C. finmarchicus at copepodite stage CV and euphausiid larva dominated the size range from 0.58 to 4 mm(3). Their growth rates increased from 0.2 d(-1) at the size of 0.71 mm(3) to the peak of 0.59 d(-1) at the size of 1.9 mm(3), and then decreased to 0.22 d(-1) at the size of 4 mm(3), In the size range above 4 mm(3), euphausiid juveniles and adults were absolutely dominant. The growth rate estimated from the bio-volume spectra varied between 0.11 and 0.19 d(-1) at the size range from 4 to 400 mm(3). In the size range between 0.3 and 0.58 mm(3), the mortality rate varied between -0.03 and -0.05 d(-1). The mortality rate reached -0.38 d(-1) in the range from 0.71 to 10 mm(3). In the size range between 10 and 40 mm(3) the rate decreased from -0.31 to -0.11 d(-1), and in the size range above 40 mm(3) the rate increased to -0.27 d(-1). The comparison with estimates from net tow samples and literature data indicates that the biomass spectrum theory affords a valuable method for estimating in situ individual growth and population mortality rates by providing high resolution in size classes and a greater degree in accuracy. It is also less labor-intensive and time-consuming than other methods. A test of this method in an advective environment needs further.