| | | 8.9 Eutrophication and phytoplankton biomass accumulation |
The influence of top-down control is obviously important for the flow of nutrients through the food chain or food web. In lakes the cascading effects through the food web by manipulating the top-down regulation is well known [313]. Top-down effects have less-known effects on marine coastal eutrophication. As most of the eutrophicated regions are in the shallow coastal zone some of the peculiarities of these ecosystems have become mixed up into the term eutrophication that is almost analogue with green or brown waters. Do green or brown waters indicate marked increases in marine productivity or eutrophication? In most cases in the coastal zone this is the case, but it could also just reflect the lack of important grazers such as copepods that are excluded from overwintering in shallow waters, resulting in decreased grazing pressure on large-celled bloom phytoplankton. Primarily brown and green waters do not suggest that there is less grazing than phytoplankton production: a mismatch between producers and consumers. Do blue water indicate that marine productivity is low? In some cases this is the case, in others no. Blue water reflects a balance between producers and consumers: biomass accumulation does not take place. Production could be high or low. Contradicting to common believe, some blue waters are highly eutrophic (e.g. the north Norwegian shelf; [491]) while others are oligotrophic (e.g. the central and eastern Mediterranean Sea). These findings have obvious implications for the interpretation of pigment data and remotely sensed pigments concentrations that traditionally have been applied to construct PT fields over regions where PT measurements were unavailable. There cannot exist a constant pigment/PT ratio, analogous to that no constant PT/PE ratio exists. The top-down regulation of phytoplankton biomass is thus important to keep in mind when the eutrophic status of a region is established. Blue water can produce large amounts of detritus and result in large-scale vertical export of biogenic matter, resulting in large supply to the benthos and oxygen deficiency in bottom waters.
The cascading effect of top-down manipulation influenced the plankton community and results in different functional response in the various regions exposed to eutrophication. During the process of eutrophication, the food web structure, timing of fertilisation and alternative grazing/predation strategies of the planktonic heterotrophs have a crucial impact on the retention and loss of nutrients from the pelagic zone [226][450].
| | | 8.9 Eutrophication and phytoplankton biomass accumulation |