Issues in Ecology Number 7 al2000 duce powerful toxins or cause other harm to humans.fisher Effects on Seagrass Beds and Corals ies resources,and coastal ecosystems (Figure 3).These spe Eutrophication frequently leads to the deoradatio cies make their in thee red or brown tides-to dilute.inconspicuous concentra availability further by stimulating the growth of phytoplank tions of cells noticed only because of damage caused by their ton,epiphytes on the seagrass leaves.and nuisa potent toxins.Impacts can indude mass mortalities of wild and of ephemeral seaweeds (macroalgae)that shade out both farmed fish and shellfish,human poisonings from contaminated seagrasses and perennial seaweeds such as kelp.In addition fish or shellfish.alterations of marine food webs through dam eutrophication can lead to elevated concentrations of sulfide 智me in the sediments of seagrass beds as algae and plant material decompose on the oxygen-depleted seafloor and seagrasses Although population explosions of toxic or noxious lose their ability to oxygenate the sediments.These elevated algal species are sometimes called red tides.they are more sulfide levels can slow the growth of seagrasses.which draw As with most algal most of their nutrients from the sediments rather than the this proliferation and occasional dominance by par water column.or even poison them and lead to their decline ticular species a combination of physical,chemi Nutrient-induced changes generally lower the bio that remair logica diversity of seagrass and kelp communities.Since these ugh ham for a or year d ce and durati rleast som Pe1972 th d exa p of the link he and lution involves the recently discov noflagellate Pfiesteria.In North Carolina estuarie and in the chesaneake bay this oroanism has bee linked to fish kills and to a variety of human effects including severe learning and memory pr lems because the organism and associated fish kills have occurred in watersheds that are heavily pol luted by hog and chicken farm wastes and by mu- nicipal sewage,a strong case can be made that nu trient pollution serves as a major stimulant to out breaks of Pfiesteria or Pfiesterialike organisms.Nu Pre trient-laden wastes could stimulate the outbreaks by either of two mechanisms. First.Pfiesteria is able to take up and use some of the dissolved organic nutri- ents in the waste directly. this adaptabl organism can cons algae that have grov VE H P the Figure 3.This shows the expansion of harmful algal blooms in the United States pre-1972 to present.Ciguatera blooms accumulate toxins in fish argely in tropical waters.Brown tides destroy shellfish beds.Pliesteria kills n tid fish and harms humans.(from Ander ish poiso hat ded reg son 1995,as reprinted in NRC 2000)
5 Issues in Ecology Number 7 Fall 2000 duce powerful toxins or cause other harm to humans, fisheries resources, and coastal ecosystems (Figure 3). These species make their presence known in many ways, ranging from massive blooms of cells that discolor the water so-called red or brown tides to dilute, inconspicuous concentrations of cells noticed only because of damage caused by their potent toxins. Impacts can include mass mortalities of wild and farmed fish and shellfish, human poisonings from contaminated fish or shellfish, alterations of marine food webs through damage to larval or other life stages of commercial fisheries species, and death of marine mammals, seabirds, and other animals. Although population explosions of toxic or noxious algal species are sometimes called red tides, they are more correctly called harmful algal blooms. As with most algal blooms, this proliferation and occasional dominance by particular species results from a combination of physical, chemical, and biological mechanisms and interactions that remain poorly understood. Although harmful algal blooms have occurred for at least thousands of years, there has been an increased incidence and duration of such outbreaks worldwide over the past several decades. This increase in harmful algal blooms has coincided with marked increases in nutrient inputs to coastal waters, and in at least some cases, nutrient pollution is to blame for the outbreaks. A frequently cited example of the suspected link between harmful algal blooms and nutrient pollution involves the recently discovered phantom dinoflagellate Pfiesteria. In North Carolina estuaries and in the Chesapeake Bay, this organism has been linked to fish kills and to a variety of human health effects, including severe learning and memory problems. Because the organism and associated fish kills have occurred in watersheds that are heavily polluted by hog and chicken farm wastes and by municipal sewage, a strong case can be made that nutrient pollution serves as a major stimulant to outbreaks of Pfiesteria or Pfiesteria-like organisms. Nutrient-laden wastes could stimulate the outbreaks by either of two mechanisms. First, Pfiesteria is able to take up and use some of the dissolved organic nutrients in the waste directly. Second, this adaptable organism can consume algae that have grown more abundant because of the nutrient over-enrichment. Although the link between Pfiesteria outbreaks and nutrient pollution has not been fully proven, the evidence is sufficiently strong that legislation is already being developed and adopted to regulate waste handling at hog and chicken farms to reduce nutrient inputs to adjacent watersheds. Pfiesteria has thus prompted some agencies to address serious and long-standing pollution discharges by nonpoint sources that had previously avoided regulation. Effects on Seagrass Beds and Corals Eutrophication frequently leads to the degradation or complete loss of seagrass beds. Plant growth in these beds is often light limited, and eutrophication can lower light availability further by stimulating the growth of phytoplankton, epiphytes on the seagrass leaves, and nuisance blooms of ephemeral seaweeds (macroalgae) that shade out both seagrasses and perennial seaweeds such as kelp. In addition, eutrophication can lead to elevated concentrations of sulfide in the sediments of seagrass beds as algae and plant material decompose on the oxygen-depleted seafloor and seagrasses lose their ability to oxygenate the sediments. These elevated sulfide levels can slow the growth of seagrasses, which draw most of their nutrients from the sediments rather than the water column, or even poison them and lead to their decline. Nutrient-induced changes generally lower the biological diversity of seagrass and kelp communities. Since these Figure 3 - This shows the expansion of harmful algal blooms in the United States pre-1972 to present. Ciguatera blooms accumulate toxins in fish, largely in tropical waters. Brown tides destroy shellfish beds. Pfiesteria kills fish and harms humans. (from Anderson 1995, as reprinted in NRC 2000). Pre-1972 Present Neurotoxic shellfish poisoning Paralytic shellfish poisoning Fish kills Ciguatera Pfiesteria Brown tide Amnesic shellfish poisoning
Issues in Ecology Number 7 Fall 2000 Figure 4-Nitrogen over-enrichment can lead to nuisance blooms of ephemeral seaweeds(macroalgae,left photo).which can have severe impacts on seagrass beds and coral reefs.On the right,sponges and corals overgrown by the seaweed Codium isthmocladum in Southeast Florida. plant communities provide food and shelter for a rich and Given the high light intensities and warm tempera- diverse array of marine animals,the degradation of seagrasses tures found in coral reef waters,the growth rates of ephem- and kelp or their replacement by nuisance seaweed blooms eral seaweeds are limited largely by the availability of essen- brings marked changes in the associated animal life.These tial nutrients.Thus even slight increases in dissolved nutrient systems are particularly important as spawning and nursery concentrations can lead to expansion of these algae at the grounds for fish.Further,the roots and rhizomes of seagrasses expense of coral.Increased seaweed cover on reefs inhibits stabilize bottom sediments,and their dense leaf canopy pro- the recruitment of corals and leads to a cascade of other motes the settling out of fine particles from the water col- ecological effects.For instance,seaweed blooms can lead to umn.Loss of seagrass coverage,therefore,allows sediments oxygen depletion on reef surface as these seaweeds decom- to be stirred up.This not only reduces water clarity directly pose,and hypoxia in turn degrades habitat needed to sup- but allows nutrients trapped in the sediment to be released port high diversity of coral reef organisms and potentially into the water column,promoting additional algal blooms. important grazers. The short-lived nuisance seaweeds that result from eutrophi- There is some evidence that N availability,in addi- cation can also wash up in enormous quantities on beaches. tion to temperature,light,and other environmental factors, creating a foul smell for beachgoers and coastal residents. may influence the“coral bleaching”phenomenon一loss of Coral reefs are among the most diverse ecosystems the algal partners known as zooxanthellae that live inside in the world,and also among the most sensitive to nutrient the cells of coral animals and nourish them-that has ex- pollution.The world's major coral reef ecosystems are found panded globally in recent years. in naturally nutrient-poor surface waters in the tropics and subtropics.It was once commonly thought that coral reefs WHICH NUTRIENTS MATTER? preferred or thrived in areas of nutrient upwelling or other nutrient sources,but this idea has been shown to be incor- The major nutrients that cause eutrophication and rect.Instead,high nutrient levels are generally detrimental other adverse impacts associated with nutrient over-enrich- to reef health and lead to shifts away from corals and the ment are N and P.Nitrogen is of paramount importance both coralline algae that help build the reef structure toward domi- in causing and controlling eutrophication in coastal marine nance by algal turfs or seaweeds that overgrow or cover the ecosystems.This is in contrast to freshwater (or non-saline) reefs.For example,some offshore reefs in the Florida Keys lakes,where eutrophication is largely the result of excess P that contained more than 70 percent coral cover in the 1970s inputs.Other elements-particularly silica-may also play now have about 18 percent coral cover;mats of algal turf a role in regulating algal blooms in coastal waters and in and seaweeds now dominate these reefs,accounting for 48 determining some of the consequences of eutrophication. to 84 percent cover,and nutrient enrichment bears much of Extensive studies in the early 1970s led to consen- the blame(Figure 4).The effects of nutrient pollution,how- sus that P was the nutrient most responsible for over-enrich- ever,can often be exacerbated either by disease or overfish- ment in freshwater lakes.Since that time,tighter restric- ing,which reduce populations of sea urchins,fish,and other tions on P inputs have greatly reduced eutrophication prob- animals that graze on algae and help keep coral reefs clear. lems in these waters.However,more recent research indi- 6
6 Issues in Ecology Number 7 Fall 2000 plant communities provide food and shelter for a rich and diverse array of marine animals, the degradation of seagrasses and kelp or their replacement by nuisance seaweed blooms brings marked changes in the associated animal life. These systems are particularly important as spawning and nursery grounds for fish. Further, the roots and rhizomes of seagrasses stabilize bottom sediments, and their dense leaf canopy promotes the settling out of fine particles from the water column. Loss of seagrass coverage, therefore, allows sediments to be stirred up. This not only reduces water clarity directly but allows nutrients trapped in the sediment to be released into the water column, promoting additional algal blooms. The short-lived nuisance seaweeds that result from eutrophication can also wash up in enormous quantities on beaches, creating a foul smell for beachgoers and coastal residents. Coral reefs are among the most diverse ecosystems in the world, and also among the most sensitive to nutrient pollution. The worlds major coral reef ecosystems are found in naturally nutrient-poor surface waters in the tropics and subtropics. It was once commonly thought that coral reefs preferred or thrived in areas of nutrient upwelling or other nutrient sources, but this idea has been shown to be incorrect. Instead, high nutrient levels are generally detrimental to reef health and lead to shifts away from corals and the coralline algae that help build the reef structure toward dominance by algal turfs or seaweeds that overgrow or cover the reefs. For example, some offshore reefs in the Florida Keys that contained more than 70 percent coral cover in the 1970s now have about 18 percent coral cover; mats of algal turf and seaweeds now dominate these reefs, accounting for 48 to 84 percent cover, and nutrient enrichment bears much of the blame (Figure 4). The effects of nutrient pollution, however, can often be exacerbated either by disease or overfishing, which reduce populations of sea urchins, fish, and other animals that graze on algae and help keep coral reefs clear. Given the high light intensities and warm temperatures found in coral reef waters, the growth rates of ephemeral seaweeds are limited largely by the availability of essential nutrients. Thus even slight increases in dissolved nutrient concentrations can lead to expansion of these algae at the expense of coral. Increased seaweed cover on reefs inhibits the recruitment of corals and leads to a cascade of other ecological effects. For instance, seaweed blooms can lead to oxygen depletion on reef surface as these seaweeds decompose, and hypoxia in turn degrades habitat needed to support high diversity of coral reef organisms and potentially important grazers. There is some evidence that N availability, in addition to temperature, light, and other environmental factors, may influence the coral bleaching phenomenon loss of the algal partners known as zooxanthellae that live inside the cells of coral animals and nourish them that has expanded globally in recent years. WHICH NUTRIENTS MATTER? The major nutrients that cause eutrophication and other adverse impacts associated with nutrient over-enrichment are N and P. Nitrogen is of paramount importance both in causing and controlling eutrophication in coastal marine ecosystems. This is in contrast to freshwater (or non-saline) lakes, where eutrophication is largely the result of excess P inputs. Other elements particularly silica may also play a role in regulating algal blooms in coastal waters and in determining some of the consequences of eutrophication. Extensive studies in the early 1970s led to consensus that P was the nutrient most responsible for over-enrichment in freshwater lakes. Since that time, tighter restrictions on P inputs have greatly reduced eutrophication problems in these waters. However, more recent research indiFigure 4 - Nitrogen over-enrichment can lead to nuisance blooms of ephemeral seaweeds (macroalgae, left photo), which can have severe impacts on seagrass beds and coral reefs. On the right, sponges and corals overgrown by the seaweed Codium isthmocladum in Southeast Florida. Photo by Brian LaPointe. Photo by Michael Bo Rasmussen.