Determination of the growth rate and elimination of certain phytoplankton species and populations in the Sevastopol Bay (Black Sea)

The relevance of the study is determined by the key role of phytoplankton in the functioning of marine ecosystems, since unicellular algae carry out the primary production of organic matter in photosynthesis and form the first trophic link in the food chain. The tasks of the work are to determine the values of the net (apparent) growth rate of phytoplankton as a whole and of its constituent elements, including populations of certain species, and to study the possibility of estimating the rate of their actual growth and consumption based on regular short-period determinations of phytoplankton abundance and biomass in natural communities. For the determinations, we used both the previously obtained data of the decadal monitoring of the phytoplankton state in the coastal zone and the results of recent experiments. The decadal determinations performed during 2007 near the mouth of the Sevastopol Bay included measurements of phytoplankton abundance and biomass, as well as of chlorophyll a concentrations at intervals of several days, which made it possible to calculate the net specific biomass growth rate (apparent growth, k) according to its changes identified during this period. By comparing the abundance of certain species during short-period determinations, 29 values of the apparent growth were obtained for 9 mass species. The apparent growth, being the difference between actual growth and consumption, can be used to determine these values. Patterns linking the values of the apparent and actual growth of algae cells with their sizes were found in the experiments conducted by the dilution method. In our experiments, we adapted classical dilution method for the study of total phytoplankton to determine the functional parameters of certain species, which made it possible to find the linking patterns. Using the found patterns, we obtained 22 values of growth rate µ and consumption rate m for 7 mass phytoplankton species at the mouth of the Sevastopol Bay. Studies show that the growth rate of certain species of phytoplankton, whose cell volume is less then 1000 µm³, can reach values over 1 day−1. For algae with the cell volume exceeding 1500 µm³, the values of µ approach the values of k, and the values of m approach zero. For this group, the actual growth rate µ is taken to be equal to the apparent growth rate k, and the rate of consumption m is taken to be zero. In fact, in the natural population, small species are predominantly consumed in the upper illuminated layer, while the larger ones sink to the bottom or into deep horizons. This means that primarily phytoplankton populations with small cells are consumed, while populations with larger cells are not consumed. However, this conclusion is not consistent with the rates of apparent growth measured in situ, which include both positive and negative values for all populations, regardless of cell size. This contradiction is explained by the fact, that in the experiment, large cells dying off and sinking to the bottom of the vessel are taken into account on a par with living cells, and it hides the effect of elimination.