Changes in the timing of phytoplankton blooms: comparison between northern and southern Adriatic

Abstract

The phenology of phytoplankton blooms holds significant implications for marine ecosystems as it shapes pelagic food webs. The onset, intensity, and duration of phytoplankton blooms, along with their synchronization with zooplankton cycles, can impact the survival rates of these species and overall community production. In this study, we employ a combination of in situ and satellite-derived chlorophyll concentrations, utilizing various statistical methods to discern the presence and timing of spring and autumn blooms in different regions of the Adriatic Sea. The northern Adriatic (NA) represents a coastal, river-dominated ecosystem influenced by anthropogenic nutrient enrichment, with a recent decline observed in chlorophyll concentration and primary production. Conversely, the southern Adriatic (SA) is characterized as a true pelagic ecosystem with minimal influence from coastal waters on nutrient levels. Here, primary production is primarily controlled by meteorological conditions that dictate convective mixing and nutrient availability for autotrophic uptake. Our analysis reveals that the northern Adriatic predominantly experiences both spring and autumn blooms, whereas the southern Adriatic witnesses only autumn blooms, peaking in late autumn or winter. We investigate trends in the timing of the onset and peak of phytoplankton blooms, searching for environmental factors influencing these shifts. As anticipated, the onset of the autumn bloom is found to be delayed, with statistically significant trends observed in specific areas. It is worth noting that the lack of statistical significance in some instances may be attributed, at least in part, to the relatively short period of available satellite data (from 1997 onwards).