Long-term variability of vertical chlorophyll a and nitrate profiles in the open Black Sea: eutrophication and climate change

Abstract

The objective of this study was to investigate long-term trends in the subsurface summer phytoplankton community and their underlying mechanisms in relation to nutrient enrichment of the open Black Sea. We analysed parameters describing the summer chlorophyll a (chl a) and annual mean nitrate depth distribution in terms of their maximum concentration and its depth, the mean nitrate gradient for the layer between the 2 maxima, and Secchi depth. Investigating the entire period from 1964 to 1996 did not reveal any direct relationship between the nitrate and phytoplankton levels. The nitrate level in the open Black Sea increased until 1984/85, corresponding with general increases in anthropogenic riverine nutrient inputs to the sea. Thus, the average maximum nitrate concentration and the nitrate gradient for all depth profiles increased between 1969 and 1984 and then remained constant. Despite the early increase in nitrate characteristics, the maximum in the chl a profile remained unaltered from 1964 to 1984. After the mid-1980s, the magnitude of the chl a profile gradually increased to 1991 and peaked with an exceptionally high concentration in 1992. In this same period there were only moderate fluctuations in the nitrate profile characteristics. The change of the summer phytoplankton biomass levels since 1984/85 coincided with an intensification of the Rim Current and the associated transport of nitrate to the euphotic zone. Prior to 1984/85 strong stratification and a deep halocline resulted in nitrate accumulation only, whereas uplifting of the halocline, combined with intensified advective currents, increased the transport of nitrate to the euphotic zone during the summer. This resulted in an associated increase in phytoplankton production and biomass in the following period. While this study shows that the open Black Sea, like the coastal regions, has been impacted by eutrophication, the response is different from the coastal region due to the different physical characteristics of the open waters.

Fluctuations of Phytoplankton Community in the Coastal Waters of Caspian Sea in 2006

“…The reduced Secchi disk depth in 2006 could be related to the increase of phytoplankton occurring during our study (57,300 cells.l −1 ; Fig. 3) as compared to 1996-1997 (Nasrollahzadeh et al, 2008b; annual average: 13,000 cells.l −1 ) and the beginning of eutrophication in coastal of marine ecosystem (Yunev et al, 2005). Unfortunately, we could not talk strongly regarding this relationship, as there are no phytoplankton biomass data during 2006.…”

“…Furthermore, the fluctuations of the phytoplankton community's relationship to environmental parameters (pollutions) and nutrient upwelling were not extensively investigated up to now Bagheri et al, 2010). Since the southern Caspian Sea is influenced to a high extent by fresh water inflow with a heavy load of artificial nutrients (Dumont, 1995;Salmanov, 1999;Yunev et al, 2005;Sharifi, 2006;CEP, 2006;Stolberg et al, 2006), it is important to assess to which extent the increased eutrophication affects the phytoplankton abundance and species composition in the coasts of Caspian Sea.…”

Feeding and ecomorphology of three clupeoids in the N Aegean Sea

“…Such a contrasting difference may be attributed to the high eutrophication of the Black Sea and therefore the high abundance of phytoplankton (e.g. Yunev et al, 2005). Thus, the feeding preference of anchovy in the Black Sea is similar to anchovies thriving in upwelling areas (see van der Lingen et al, 2009 and references therein).…”

Phytoplankton responses to nutrient status: application of a screening method to the northern Baltic Sea

“…Nutrient enrichment of coastal waters has generally led to increases in phytoplankton biomass and primary production, although physical and biological features of the ecosystem may modulate the response (Cloern 2001, Yunev et al 2005. Changes in phytoplankton composition have often been linked with increasing nutrient inputs and particularly to altered nutrient ratios (Cloern 2001).…”