Indirect Effects in Aquatic Systems

Awareness of indirect interactions in aquatic environment has rather a considerably long history, and clearly presented examples can be found in works (among others) of, for example, Mortimer, Hutchinson, and Reynolds. In particular, in an earlier review by Abrams it was even suggested that most studies specifically addressing behavior-mediated indirect effects tend to be conducted in freshwater ecosystems, while many of the early demonstrations of density-mediated indirect effects were done in community studies in marine habitats. Likewise, much of the knowledge related to indirect ecological interactions has been contributed through the development and applications of the methods of simulation modeling and network analysis in relation to aquatic environment. Consequently, simulation models capable of demonstrating indirect interactions in aquatic biogeocenoses (e.g., the Lake 2 model of J. Solomonsen) are widely used for teaching in the educational establishments across the world.

Recent studies of indirect effects in aquatic environment variously involved a combination of the empirical approach and an application of statistical techniques, methods of network analysis, simulation modeling using 'What if' scenarios, and sensitivity analysis. One of the perhaps most frequently addressed examples of indirect effects in aquatic environment relate to trophic cascades, which involve propagation of the effect along a vertical trophic chain consisting of three or more components connected by grazing or predation. For instance, as was recently investigated by Daskalov, a decrease in the top predator's population in the Black Sea due to overfishing resulted in a 'trophic casade', leading to an increase in the abundance of planktivorous fish, a decline in zooplankton biomass, and an increase in phytoplankton crop.

The previously made statements regarding the abiotic components (see above) can be emphasized with examples related to the importance of detritus. For instance, Carrer and Opitz found that in the Lagoon of Venice about half of the food of nectonic benthic feeders and nectonic necton feeders passed through detritus at least once, while there was no direct transfer of such food according to the diet matrix. Whipple provided an analysis of the extended path and flow structure for the well-documented oyster reef model. Few simple paths and large number of compound paths were counted. The study provided structural evidence for feedback control in ecosystems, and illustrated importance of nonliving compartments (in this case, detritus) for the ecosystem's functioning. Even for the model with a low cycling index (i.e., 11%) multiple cyclic passage paths provided a considerable (22%) flow contribution. Therefore, it was envisaged that for ecosystems with higher cycling indexes the patterns observed should be even more pronounced.

Another noteworthy illustration of indirect effects in aquatic ecosystems relates to the interdependency of bio-geochemical cycles. For example, Dippner concluded that indirect effect of the silicate reduction in coastal waters causes an increased flagellate bloom, due to a high availability of riverborne nutrient loads. In a study of lake Suwa (Japan), Naito and co-authors have shown that the physiological parameters of the diatom Melosira were the important sources of the cyanobacterium Microcystis' production variability. These results agree well with our work on Rostherne Mere and suggest that the underlying mechanism might be a common inverse relationship between spring diatom and summer cyanobacterial blooms resulting from the fact that the biogeochemical cycles of Si and P in the aquatic environment are coupled via the dynamics of primary producers (i.e., increased concentrations of Si in spring lead to an increase in a spring diatom bloom, and an increase in the removal of P, N, and microelements from the water column with easily sedimenting biomass at the end of the bloom; consequently, this may lead to a decrease in the summer cyanobacterial development).

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