Plankton composition and dynamics in coastal waters around Gibraltar

Abstract

Environmental conditions are key in driving biological changes within any given community. Phytoplankton communities are no different. Therefore, understanding the physical parameters which are driving oceanic chemistry within the Straits of Gibraltar and coastal waters of Gibraltar is fundamental in understanding the changes being observed in the biological communities. Although much work has been done to try and unravel the drivers of physical and chemical changes, further work is required to better understand exactly how these changes manifest themselves in an unpredictable stretch of water like the Straits of Gibraltar. This thesis reviews much of the work that has been accumulating over the last quinquagenary and attempts to collate and discern potential implications of these findings within the Straits of Gibraltar. To establish a broad scale baseline for the coastal waters around Gibraltar a 24-year satellite data series was compiled to establish climatology’s on sea surface temperature, chlorophyll, photosynthetically active radiation, and the attenuation co-efficient of photosynthetically active radiation to create a larger scale understanding of environmental trends. This work is important to contextualise the in situ data which has been collected. The in situ data allows more fine analysis in the southern British Gibraltar territorial waters and adds a depth component to it. This is important for understanding how the water column changes seasonally and for measuring how large these changes are. By obtaining fluorescence measurements from chlorophyll and phycoerythrin it was possible to establish a relationship between the two. The ratio of the two pigments was compared with others obtained in the literature and then a physiologically based ratio was applied to estimate the contribution of cyanobacteria chlorophyll to the total community chlorophyll and investigate community composition. Predation in the form of grazing, is an important top-down pressure in marine ecosystems which controls community composition. This pressure can be selective on a small number of species or it can be broad, which affects the whole community. But who grazes the grazers and what are the possible trophic effects within an ecosystem? In order to explore this question, the predation of grazers is considered by investigating how jellyfish can alter a population of grazers of zooplankton.