Mounting threat of harmful algal bloomsHarmful algal blooms (HABs) are massive growths of toxic or otherwise harmful phytoplankton. HABs are becoming ever more frequent worldwide. It is not yet known, however, why this is. Eutrophication, the increased concentration of nutrients in the water, is considered to be the main cause, but climate change also appears to play a role. Harmful algal blooms normally occur in the summer months when the water column is thermally stratified. A warm, light surface layer overlies a colder, heavier deep layer. The warmer the surface water, the more pronounced the temperature gradient is at the thermocline between the layers.
5.6 > The mauve stinger, Pelagia noctiluca whose painful sting is normally not deadly to humans, has become increasingly abundant in the Mediterranean Sea in recent years.
5.7 > The dinoflagellate Karenia brevis occurs primarily in the Gulf of Mexico. Its nerve poison, Brevetoxin A, can lead to inflammation and asthma attacks in humans.
5.8 > Small crustaceans like this Calanus-species are widely distributed in the oceans and one of the most important food sources for fish.
- A strong temperature gradient prevents water masses from mixing at the thermocline, because the density difference between the cold and heavy water, and the warm and light water, acts as a barrier. Nutrients from greater depths are therefore prevented from circulating to the surface. So when the nutrients near the surface have been consumed by phytoplankton growth, there is no source of replenishment. The vertical barrier between a zone with enough light and insufficient nutrients, and a zone with insufficient light and abundant nutrients, which is characteristic of the summer, is thus reinforced. Relatively large mobile phytoplankton species have an advantage here. With vertical migration they can move back and forth from the deeper nutrient-rich water to the shallower layer penetrated by light where photosynthesis is carried out. Such species include numerous dinoflagellates and, especially in the Baltic Sea, cyanobacteria, which can regulate their specific gravity to rise and descend like a diver. One problem with this is that both groups include numerous toxic species. If mussels consume these organisms, then the mussels become dangerous or even lethal to humans. The planktonic organisms may also release some of the toxins directly into the water. In some cases these are even detectable in aerosols, small droplets wafting in the air that are produced by breaking waves in the surf. An especially notorious culprit is the dinoflagellate Karenia brevis, whose periodic blooms off the coast of Florida cause mortality of fish, poisoning of mussels, inflammation in swimmers and, in extreme cases, asthma attacks in visitors to the beach. Experts attribute the increased incidence of these blooms to general climate warming. As mentioned, there are also numerous toxic groups of cyanobacteria. Investigations so far have focused on cyanobacteria that live in freshwater – especially in waters that are used as sources for drinking water or where bathers are in danger from cyanobacteria. But toxic strains of various cyanobacteria such as Nodularia spumigena have also been verified in the Baltic Sea.
Trouble with jellyfishBeside more frequent HABs scientists are also observing explosive growth of jellyfish populations. The impacts of such proliferation are quite well known: injured swimmers, clogged fish nets, feeding competition for fishes, and predation of fish eggs and larvae. The possible causes of these burgeoning populations are somewhat controversial. One significant problem, presumably, is overfishing. Fish that feed on zooplankton are in feeding competition with the jellyfish. If the fish are absent, then the jellyfish have an abundance of available food. It is also known that jellyfish are more robust than many species of fish, especially in the sense that they can tolerate much lower oxygen concentrations. Oxygen deficiencies in the oceans, in turn, occur increasingly as a result of eutrophication. More biomass is created because of eutrophication. As a result, more organic material sinks into the deep water where it is decomposed by oxygen-consuming microorganisms. The result is a general decrease in oxygen. Climate change, which causes warming of the ocean surface, can exacerbate this situation. The warming slows down exchange processes because the oxygen-rich surface water mixes less with the colder deep water. Only small amounts of the oxygen consumed by microorganisms at greater depths are replaced. Increasing jellyfish scourges could therefore be a result of combined stressors. Climate change will therefore lead to a restructuring of the pelagic biocoenoses, which will disadvantage the classical food chain “phytoplankton – zooplankton – fish”. Jellyfish, on the other hand, will benefit from that. Presumably overfishing and eutrophication of coastal waters will have additional synergistic effects that will worsen the situation.