Ecosystems

Over the past century, a wide range of human activities—the intensification of agriculture, waste disposal, coastal development, and fossil fuel use—has substantially increased the discharge of nitrogen, phosphorus, and other nutrients into the environment. These nutrients are moved around by streams, rivers, groundwater, sewage outfalls, and the atmosphere and eventually end up in the ocean. Once they reach the ocean, nutrients stimulate the growth of tiny marine plants called phytoplankton or algae. When the concentration of nutrients is too high, this growth becomes excessive, leading to a condition called eutrophication.

In some environments, there is a connection between eutrophication, harmful algal blooms (HABs), and the depletion of oxygen dissolved in bottom waters (hypoxia). When vertical mixing is weak or absent, oxygen-depleted bottom waters are not refreshed, resulting in a condition called hypoxia. Hypoxic areas—popularly known as “dead zones”—occur regularly in Long Island Sound, the Chesapeake Bay, and the northern Gulf of Mexico.

The indirect effects of hypoxia can be substantial. Suitable habitat is lost, putting pressure on populations. Hypoxic zones also can interfere with the migratory behavior of shrimp, lobsters, and other species. More generally, by altering the environment in which marine species thrive, hypoxia can lead to a decline in biological diversity. There is evidence that the hypoxia off the coast of Louisiana has harmed the valuable shrimp fishery and possibly contributed to the replacement of bottom-dwelling species such as snapper with less valuable mid-water species such as menhaden.

Native green seaweed Codium isthmocladum formed a bloom on deep reef communities in eastern Florida. (B. LaPointe)

Blooms of opportunistic macroalgae (e.g., seaweeds such as red algae Laurencia intricata and Spyridia filamentosa; brown algae, Dictyota spp. and Sargassum filipendula; and green algae, Enteromorpha spp., Codium isthmocladum, and Halimeda spp.) have caused problems in many coastal regions.  These species out-compete, overgrow, and replace seagrass and coral reef habitats, resulting in reduced light availability to bottom communities, lower productivity, habitat loss from low oxygen conditions, and eventual die-off of sensitive species.  Some macroalgal blooms have been linked to nutrient enrichment of coastal waters.  Some invasive macroalgae are responsible for multiple adverse impacts:  not only are they able to out-compete seagrass and more desirable species, but some may also be toxic, such as Lynbya majescula, and exposure can lead to skin rashes or other serious problems for humans or animals.

In southeast Florida, macroalgal HABs have impacted coral reefs for the past two decades.  These phenomena began during a regional drought in 1990-1991 when the native green seaweed Codium isthmocladum formed massive summer blooms on deep reef communities in eastern Florida.  The excessive biomass of these HABs resulted in hypoxia and anoxia in hear-bottom waters, causing die-off of hard and soft corals, sponges, and other reef biota, resulting in an overall decrease in abundance and diversity of reef fishes.  This was followed by expansive blooms of several Caulerpa spp. in 1997 and 2001. Although summertime upwelling can provide natural nutrient subsidies to these macroalgal HABs, stable nitrogen isotope and water column nutrient data suggest a linkage to land-based discharges of both wastewater (septic tanks, injection wells, ocean outfalls) and agricultural runoff.

Ecosystem impacts stemming from the effects of both toxic and non-toxic high biomass cyanobacterial blooms are well documented.  Hypoxic events that suffocate and kill fish and bottom-dwelling organisms are perhaps the most common adverse impact of high biomass cyanoHAB blooms.  In addition, blooms can block sunlight penetration into the water column, preventing growth of beneficial algae.  This in turn disrupts aquatic food webs, as cyanobacteria are comparably unpalatable and of low food quality for grazers.  These conditions can lead to die-offs of marine organisms at lower trophic levels that echoes up the food chain, which can result in starvation of consumers and their predators.

The toxicity associated with many cyanoHABs can exacerbate the nature of these impacts on aquatic ecosystems, but the importance of toxicity relative to the stressors described above is unclear. Cyanotoxins can accumulate in the primary consumers and potentially be transferred up the food web.  Cyanotoxins have been implicated as the cause of mass mortalities of fish and birds and have also been tied to the death of pets and livestock which may be exposed through drinking contaminated water or licking themselves after bodily exposure.  Furthermore, ungrazed cyanobacterial biomass that accumulates as clotted mats pose a particular threat to dogs, who may eat the toxic mats.  For this reason, dog deaths have emerged as an unfortunate early warning that a toxic cyanobacterial bloom is occurring. The impacts of cyanoHABs are not limited to freshwater systems, as for example, a 2007 cyanobacteria bloom in California rivers was carried out into Monterey Bay, resulting in the deaths of several dozen sea otters.

Brown tides are a HAB phenomenon caused by the microalga Aureococcus anophagefferens, the cause of the mid-Atlantic brown tide, and a related species, Aureoumbra lagunensis, the cause of brown tides along the Texas coastline. Both algae are typically found in nearshore waters in low concentrations, but when environmental and ecological conditions align, the algae can produce a bloom in excess of billions of cells per liter. An individual cell of either algae is a light golden color; a massive bloom turns ocean waters brown and completely opaque.

When the bloom is sufficiently dense, no sunlight reaches the benthic ecosystem, killing off beds of seagrass that serve as spawning grounds for shellfish and native fishes. The loss of this habitat can devastate marine populations, as well as local fishing and shellfishing harvests for years after the event. In addition to the loss of spawning habitat, Aureococcus particularly can create hypoxic "dead zones", where the explosive growth of the algae depletes the oxygen in the water to the detriment of all other species. These can result in massive fish kills, and ensuing damage to local fishing and tourist industries. For example, brown tides have caused mass mortalities of mussel populations in Rhode Island and in Long Island waters. Recurrent blooms have also had severe impact on bay scallops, affecting more than 80% of New York's commercially valuable harvest.