Human activities are having profound effects on marine ecosystems. Since the mid 20th century, marine food webs have undergone dramatic changes due to the emergence of industrial fishing. Humans tend to feed on fish with “high trophic status”- those that are near the top of the food chain. In many systems, top predators have been severely reduced or even lost entirely due to overfishing. More recently, increased nutrient loads due to agriculture are fertilizing marine ecosystems, particularly in the more delicate and high-productivity coastal habitats. This often results in explosions of phytoplankton, the tiny photosynthetic bacteria that forms the bottom of most marine food chains. In recent years these phytoplankton blooms have been further facilitated by warmer sea surface temperatures and increased runoff levels due to climate change. Taken together, overfishing and anthropogenic nutrient loading are resulting in both top-down and bottom-up forcing that is fundamentally altering the the structure of marine communities.
A recent study in Global Change Biology examined the impacts of eutrophication and overfishing on a marine community in the Black Sea, a land-locked basin in Eastern Europe. Marine communities in the Black Sea are largely isolated, and offer ecologists the opportunity to study how removal of key organisms in a food web affect the broader community. Decades of overfishing have led to a loss of several traditional apex predators, which has disturbed the structure of the system from the top down. Recent eutrophication has further altered community structure. In particular, once-restricted jellyfish species have been able to make inroads, in some cases coming to dominate the oxygen-depleted waters that result from phytoplankton blooms.
The native marine community at this research group’s study site had difficulty adapting to increased productivity at the bottom of its food web. The researchers concluded that the community as a whole would likely have fared much better if top predators had not also been removed. The inclusion of top predators would have prevented jellyfish and other minor players from becoming dominant and in turn pushing out other mid-trophic level species. These results indicate that ecosystems-based fisheries management must take into account the role of top predators in structuring communities and offering resilience to other profound changes such as nutrient loading and warming. Whether the Black Sea will recover from the major disturbances it has suffered, or revert to a low-diversity, eutrophic state will depend in large part on the economic and political decisions of the countries whose fishing and agriculture practices have so profoundly affected it.
Llope et al. 2011. Overfishing of top predators eroded the resilience of the Black Sea system regardless of the climate and anthropogenic conditions. Global Change Biology 17: 1251-1265.