PerspectiveClimate

Blooms Like It Hot

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Science  04 Apr 2008:
Vol. 320, Issue 5872, pp. 57-58
DOI: 10.1126/science.1155398

Nutrient overenrichment of waters by urban, agricultural, and industrial development has promoted the growth of cyanobacteria as harmful algal blooms (see the figure) (1, 2). These blooms increase the turbidity of aquatic ecosystems, smothering aquatic plants and thereby suppressing important invertebrate and fish habitats. Die-off of blooms may deplete oxygen, killing fish. Some cyanobacteria produce toxins, which can cause serious and occasionally fatal human liver, digestive, neurological, and skin diseases (14). Cyanobacterial blooms thus threaten many aquatic ecosystems, including Lake Victoria in Africa, Lake Erie in North America, Lake Taihu in China, and the Baltic Sea in Europe (36). Climate change is a potent catalyst for the further expansion of these blooms.

Undesired blooms.

Examples of large water bodies covered by cyanobacterial blooms include the Neuse River Estuary, North Carolina, USA (top) and Lake Victoria, Africa (bottom).

CREDITS: (TOP) HANS PAERL/UNIVERSITY OF NORTH CAROLINA; (BOTTOM) SATELLITE PHOTO, DIGITALGLOBE

Rising temperatures favor cyanobacteria in several ways. Cyanobacteria generally grow better at higher temperatures (often above 25°C) than do other phytoplankton species such as diatoms and green algae (7, 8). This gives cyanobacteria a competitive advantage at elevated temperatures (8, 9). Warming of surface waters also strengthens the vertical stratification of lakes, reducing vertical mixing. Furthermore, global warming causes lakes to stratify earlier in spring and destratify later in autumn, which lengthens optimal growth periods. Many cyanobacteria exploit these stratified conditions by forming intracellular gas vesicles, which make the cells buoyant. Buoyant cyanobacteria float upward when mixing is weak and accumulate in dense surface blooms (1, 2, 7) (see the figure). These surface blooms shade underlying nonbuoyant phytoplankton, thus suppressing their opponents through competition for light (8).

Cyanobacterial blooms may even locally increase water temperatures through the intense absorption of light. The temperatures of surface blooms in the Baltic Sea and in Lake IJsselmeer, Netherlands, can be at least 1.5°C above those of ambient waters (10, 11). This positive feedback provides additional competitive dominance of buoyant cyanobacteria over nonbuoyant phytoplankton.

Global warming also affects patterns of precipitation and drought. These changes in the hydrological cycle could further enhance cyanobacterial dominance. For example, more intense precipitation will increase surface and groundwater nutrient discharge into water bodies. In the short term, freshwater discharge may prevent blooms by flushing. However, as the discharge subsides and water residence time increases as a result of drought, nutrient loads will be captured, eventually promoting blooms. This scenario takes place when elevated winter-spring rainfall and flushing events are followed by protracted periods of summer drought. This sequence of events has triggered massive algal blooms in aquatic ecosystems serving critical drinking water, fishery, and recreational needs. Attempts to control fluctuations in the discharge of rivers and lakes by means of dams and sluices may increase residence time, further aggravating cyanobacteria-related ecological and human health problems.

In addition, summer droughts, rising sea levels, increased withdrawal of freshwater for agricultural use, and application of road salt as a deicing agent have led to rising lake salinities in many regions. Several common cyanobacteria are more salt-tolerant than freshwater phytoplankton species (12, 13). This high salt tolerance is reflected by increasing reports of toxic cyanobacterial blooms in brackish waters (2, 6).

Some cyanobacteria have substantially expanded their geographical ranges. For example, Cylindrospermopsis raciborskii—the species responsible for “Palm Island mystery disease,” an outbreak of a severe hepatitis-like illness on Palm Island, Australia (4)—was originally described as a tropical/subtropical genus. The species appeared in southern Europe in the 1930s and colonized higher latitudes in the late 20th century. It is now widespread in lakes in northern Germany (14). Similarly, the species was noted in Florida almost 35 years ago and is now commonly found in reservoirs and lakes experiencing eutrophication in the U.S. southeast and midwest (2). It is adapted to the low-light conditions that typify eutrophic waters, prefers water temperatures above 20°C, and survives adverse conditions through the use of specialized resting cells (14). These bloom characteristics suggest a link to eutrophication and global warming.

More detailed studies of the population dynamics in cyanobacterial blooms are needed. For example, competition between toxic and nontoxic strains affects the toxicity of cyanobacterial blooms (15). Furthermore, viruses may attack cyanobacteria and mediate bloom development and succession (16). It is unclear how these processes are affected by global warming. What is clear, however, is that high nutrient loading, rising temperatures, enhanced stratification, increased residence time, and salination all favor cyanobacterial dominance in many aquatic ecosystems. Water managers will have to accommodate the effects of climatic change in their strategies to combat the expansion of cyanobacterial blooms.

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