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Environmental Consequences

 

The decrease of dissolved oxygen levels in coastal marine ecosystems around the world is cited as one of the most severe changes in ecological variables in the past decades. These hypoxic environments are increasing due to continued eutrophication, which spurs large growths of primary producers and in turn decimates dissolved oxygen levels as the organic matter decays, as well as global temperature warming, which decreases oxygen solubility, and affects ocean stratification. The conditions that these dead zones pose put marine ecosystems at great environmental risks like loss of biodiversity, massive death of aquatic life, and alterations of food webs. (Diaz et al. 2001) (Vaquer-Sunyer and Duarte et. al. 2008)

(Vaquer-Sunyer and Duarte et. al. 2008)

 

Hypoxic Ecosystems

 

In normal aquatic ecosystems, oxygen dissolves into the water directly from the atmosphere or from the respiration of phytoplankton. This dissolved oxygen supplies life to the animals on the surface waters as well as bottom waters as the oxygen mixes downward. However, in an ocean dead zone, this supply of dissolved oxygen to the bottom water (benthic zone) is decreased or decimated as the decomposition takes up oxygen..

(Addison, 2004)

 

Oxygen levels in this environment reach critical levels, which affect fish and bottom feeders like shrimp, crabs, and oysters.

 

Consequences

 

The algae blooms and the hypoxic environment have serious consequences on the marine ecosystem, including death of marine life, harm to reproduction and growth of marine life, forced changing of migration patterns, and toxicity of water.

 

Hypoxia kills off significant amounts of animals, specifically those that dwell on the benthic zone most affected by hypoxia, such as: clams, oysters, shrimp, crabs and lobsters. This leads to a serious loss of biodiversity in the marine ecosystem. However, it is difficult to tell exactly what level of dissolved oxygen leads to death since each animal reacts differently to the changing levels of dissolved oxygen. Extensive research has been completed regarding the study of specific animals and the survival threshold of each as well as the levels of dissolved oxygen that harms the development of each animal (Vaquer-Sunyer and Duarte et. al. 2008). It is hard to estimate an exact minimum amount of dissolved oxygen that leads to mass extinction, but studies show that effects on marine life begin to occur as oxygen levels go below 2 mg O₂ per Liter (Diaz et al. 2001). However, a recent study, Thresholds of hypoxia for marine biodiversity, addresses the variability of oxygen thresholds of different marine animals.

 

 

Figure: ìBox plot showing the distributions of oxygen thresholds among taxa for median lethal concentration (A), median sublethal concentration (B), and median lethal time (C)î (Vaquer-Sunyer and Duarte et. al. 2008)

 

This study illustrates that crustaceans are extremely sensitive to the reduction of dissolved oxygen compared to other marine life. They have the highest median lethal concentration and the shortest median lethal time. Mollusks show the most resistance to hypoxic conditions since their median lethal concentration is quite low. Another interesting observation is that fish are the most susceptible to sublethal concentrations, meaning that fish are significantly affected by ìreduced growth and reproduction.î The study concludes that the dangers of hypoxia affect marine life differently due to the disparities in adaptation and mobility as well as ìbehavioral and metabolic changes.î (Vaquer-Sunyer and Duarte et. al. 2008)

 

Other than affecting reproduction and growth in marine life, hypoxia causes many other stresses. In general, these areas are unsuitable habitats, leading to forced migration away from dead zones. This can lead to great strains on the animals that are not adapted to migrate long distances or are forced to live in new environments closer to the surface of the ocean.

 

All of the mortality and the devastation of life of crustaceans, fish, shrimp and crabs have larger environmental consequences within the fish population and in turn the fishing industry. Ocean dead zones in conjunction with habitat destruction and overfishing have lead to devastation fish population.

 

Table: ìPercent decline (biomass, catch, percent cover) for fauna and flora from various marine environmentsî (Jackson et al. 2008)

 

Taxon                                              Starting date                  Location             % loss

Estuaries and coastal seas

Large whales                                   Pristine                         Global                85%

Small whales                                   Pristine                         Global                59%

Pinnipeds and otters                        Pristine                         Global                55% 

Sirenia                                             Pristine                         Global                90% 

Raptors                                            Pristine                         Global                79% 

Seabirds                                          Pristine                         Global                57% 

Shorebirds                                       Pristine                          Global                61% 

Waterfowl/waders                           Pristine                         Global                58% 

Sea turtles                                        Pristine                         Global                87%

Diadromous fish                             Pristine                         Global                81%

Groundfish                                      Pristine                         Global                62% 

Large pelagics                                 Pristine                         Global                74% 

Small pelagics                                 Pristine                         Global                45% 

Oysters                                            Pristine                         Global                91% 

Mussels                                           Pristine                         Global                47%

Crustaceans                                     Pristine                         Global                39% 

Other invertebrates                          Pristine                         Global                49% 

Seagrass                                          Pristine                         Global                65% 

Wetlands                                         Pristine                         Global                67% 

Large carnivores                              Pristine                         Global                77% 

Small carnivores                              Pristine                         Global                60% 

Large herbivores                             Pristine                         Global                63% 

Small herbivores                             Pristine                         Global                54%

Suspension feeders                         Pristine                         Global                68% 

Shelf and pelagic fisheries

Large predatory fishes                     1900                             N. Atlantic         89%

Atlantic cod                                     1852                             Scotian shelf      96%

Fish 4–16 kg                                   Pristine                         North Sea           97%

Fish 16–66 kg                                 Pristine                         North Sea           99%

Large predatory fish                        1950s                            Global                90%

Large pelagic predators                   1950s                            Tropical Pacific 90%

Fishery biomass                              1959                             Bohai Sea           95%

 

Excess algae blooms lead not only to hypoxic environments but also to toxic algae. Certain algae blooms release toxic compounds, such as neurotoxins and hepatotoxins into the marine ecosystem. Often, these toxins biomagnify, meaning that they increase in concentration, as they move along in the food chain. This can threaten larger marine life such as fish and shellfish as well as humans that consume these animals. (Water quality. updated 2008)

In conclusion, marine ecology is very delicate, and hypoxic dead zones ñ its algae blooms and dissolved oxygen content ñ are a threat to these fragile ecosystems as well as humans that rely on the coast for food and life.