Long-term changes in nutrient supply of phytoplankton growth in the Black Sea

• 40 years' dynamics of phytoplankton in the open waters of the Black Sea was analyzed.
• 3 periods with different eutrophication status were identified.
• 40 years’ variations in nutrients content and their ratios were analyzed both for sub-euphotic zone and the pycnocline.
• Changes in biomass and taxonomic structure of phytoplankton were compared with dynamics of N, P, Si and their ratios.
• Difference in content and ratios of available nutrients between 3 periods has led to growth of different groups of algae.

During the period from 1970s to 2000s in the open waters of the Black Sea ecosystem has passed the stage of eutrophication, which manifested in high values of chlorophyll a, biomass of phytoplankton, primary production, etc. The goal of current research is to trace the long-term (1969–2008) dynamics in supply of nutrients and their ratios in the pycnocline, as well as changes in intensity of the upward flow of nutrients into the euphotic zone and impact of these factors on abundance and the taxonomic structure of phytoplankton. Based on changes of the total phytoplankton biomass three periods were revealed: Pre-eutrophication (PR) in 1969–1983, Eutrophication (EU) in 1984–1995 and Post-eutrophication (PS) in 1996–2008. The cold intermediate layer (CIL) in the Black Sea is formed by winter convection and store attributes of its intensity during the subsequent period of the year. Based on this, changes of nutrient content in this layer, as well as in the upper layer of the main pycnocline (Pycnocline) were used to track long-term variations of the annual bottom-up flux of nutrients to the euphotic zone. Data on nutrient content (P-PO4−, N as N-NO3− + N-NH4+ and Si) and their ratios were analyzed both for the CIL and the Pycnocline. Averaged depth-integrated phytoplankton biomass increased 6 times from 3.3 g m− 2 in PR to 19.6 g m− 2 during EU and decrease down to 10.6 g m− 2 in PS, what was still 3 times higher than during PR. Similar changes were observed in N both in the CIL and in the Pycnocline. N in the CIL averaged 1.2, 3.3 and 2.5 μM in PR, EU and PS, respectively. P has increased significantly on 30% from 0.28 μM during PR to 0.37 μM in PS, while Si sharply decreased from 20 μM in PR to 12.1 μM in PS. Nutrient ratios changed similarly in the CIL and in the Pycnocline. In the CIL, N:P ratio increased from 6.4 in PR to 11.2 in EU and then dropped down to 8.8 in PS. Si:N ratio decreased from PR to PS from 19.6 to 7.3. Different algae groups predominated in the total phytoplankton biomass: dinoflagellates in PR (58%), diatoms in EU (47%) and both diatoms and coccolithophorids (31% and 28%) in PS. Comparative analysis of nutrient dynamics and changes in taxonomic structure of phytoplankton communities showed dependence of dinoflagellates and diatoms on high N and N:P ratios. Decreasing Si did not affect diatoms while probably it limited a growth of silicoflagellates. Increase of abundance of coccolithophorids during recent decades was governed by high P.