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- 66 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 pro￾motes the settling out of fine particles from the water col￾umn. 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 eutrophi￾cation 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 world’s 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 incor￾rect. 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 domi￾nance 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, how￾ever, can often be exacerbated either by disease or overfish￾ing, 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 tempera￾tures found in coral reef waters, the growth rates of ephem￾eral seaweeds are limited largely by the availability of essen￾tial 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 decom￾pose, and hypoxia in turn degrades habitat needed to sup￾port high diversity of coral reef organisms and potentially important grazers. There is some evidence that N availability, in addi￾tion 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 ex￾panded globally in recent years. WHICH NUTRIENTS MATTER? The major nutrients that cause eutrophication and other adverse impacts associated with nutrient over-enrich￾ment 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 consen￾sus that P was the nutrient most responsible for over-enrich￾ment in freshwater lakes. Since that time, tighter restric￾tions on P inputs have greatly reduced eutrophication prob￾lems in these waters. However, more recent research indi￾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. Photo by Brian LaPointe. Photo by Michael Bo Rasmussen.