Decades of studies have indicated that climate change and eutrophication are altering the dissolved oxygen concentration in the ocean. As a result of climate change, the surface temperature of ocean increases, leading to lower oxygen solubility. Also, the warmer, more buoyant surface layer disrupts the gas exchange between the surface and the ocean floor. Such low-oxygen, or hypoxic condition in the ocean floor is becoming increasingly prevalent in various marine ecosystems. A study using the subsea observatory by the School of Earth and Ocean Science in University of Victoria aimed to understand the impact of dissolved oxygen levels on species diversity. A year-long study that took place from October 2009 to October 2010 was published on PLOS One in September 2012. The researchers concluded that hypoxia decreases species diversity and favors community of little commercial importance.
Marjolaine Matabos and Courtney Dean performed the underwater study in Saanich Inlet using VENUS (Victoria Experimental Network under the Sea), which consists of numerous imaging devices and instruments that allowed the team to gather vast amount of data on the ocean floor. The team observed significant correlation between changes in the species composition of the ocean floor and the seasonal fluctuation of dissolved oxygen. Images captured showed population of sulphide-oxidizing bacterium Beggiatoa forming a thick “mat” on the ocean floor during hypoxic conditions. These mats overly favor shrimp Spirontocaris sica, one of the three dominant species identified under hypoxic conditions. The other two species that thrived are the galatheid crab Munida quadrispina and the flatfish Lyopsetta exilis.
Beyond the mere domination of the three species lies a bigger implication for the marine ecosystems. The three species significantly dominated the ocean floor. Fish, least tolerant of the hypoxic conditions, followed by echinoderms and crustaceans, were all outcompeted by the three dominant species. The victims include commercial crustacean species like the spot prawn and the Dungeness crab, which could not tolerate severe hypoxia. In addition to negatively affecting the fishing industry, the hypoxic-tolerant species could dramatically alter the marine ecosystem. These species often occupy very different ecological roles, or niches. Therefore, they provide very different ecosystem services, if any, than the native species. There could also be significant geochemical changes following the disappearance of native species that contribute to bioturbation, or reworking of the ocean floor sediment. Furthermore, hypoxia was a form of habitat destruction. Many organisms were pushed to shallower water due to the lack of oxygen on the ocean floor. These changes could have domino effects that devastate various marine ecosystems.
Many challenges confront the scientists studying communities in the deep sea. It was difficult to collect consistent daily information on changes in community dynamics. The equipment used such as the camera tripod could have altered the behavior of species. Another challenge was the lack of visibility caused by the resuspended sediments. Visibility problem made it difficult to accurately count the species captured on camera. The study was also very costly and time-consuming.
While the Saanich Inlet provided valuable insights to the response of ocean floor community to hypoxia, more detailed study is needed to understand the precise community dynamics and the mechanisms that dictate the cascade of events due to hypoxia. Scientists could better forecast the consequences of climate change on oceanic communities, thus allowing them prioritize conservation efforts. In the meantime, steps should be taken to combat climate change.
Matabos M, Tunnicliffe V, Juniper SK, Dean C (2012) A Year in Hypoxia: Epibenthic Community Responses to Severe Oxygen Deficit at a Subsea Observatory in a Coastal Inlet. PLoS ONE 7(9): e45626. doi:10.1371/journal.pone.0045626