The Damage: Marine Systems

Human impacts do not stop with freshwater, but continue on into our oceans, often transported by the rivers themselves. These issues often arrive in the form of excess nutrients (nitrogen and phosphorous, generally) and literal garbage. Carbon emissions, which are delivered from the atmosphere rather than tributaries, also severely impact the ocean by increasing its' acidity. These issues are among others threatening the livelihood of our oceans, including habitat loss, overfishing, and other environmental problems (Diaz et al., 2008).


Oceanic Dead Zones


Eutrophication, or the addition of nutrients from the environment, shows its' first effects as water turns green due to the enhanced production of plants and algae (depicted in the following image). At this time, dissolved oxygen levels begin to decrease due to slowed or even halted photosynthesis by the organisms below the overproducing (and sunlight blocking) top layer. Many planktonic algae are unable to survive under these circumstances, and are incorporated  into the organic seabed layer and the microbial respiration that occurs there. As the dissolved oxygen concentration decreases to hypoxic conditions of around 2ml of O2/liters or less, benthic organisms begin to react by leaving their burrows and risking exposure, forming "dead zones." Then, at about 0.5ml of O2/liter (severe hypoxia) mass mortality ensues (Diaz et al., 2008).

A dead zone in the Louisiana Delta, science.nasa.gov

As organic matter and nutrients build up over time, hypoxic conditions can begin to exhibit a seasonal periodicity based on the large fluctuations among animal populations it causes. If such hypoxic events persist in regions for years, the zone can begin to expand further into the ocean, and if dissolved oxygen levels continue to decrease, H2S is released by microbes as anoxic (no oxygen) levels are reached. This is problematic because when hypoxia becomes severe on a seasonal basis, only benthic species of smaller size, shorter lifespan, and opportunistic behaviors are favored (Diaz et al., 2008).
Oxygen depletion along coasts is tightly associated with concentrated human populations and large watersheds (illustrated in the following image). Since the nitrogen fertilizer revolution in the 1940s, eutrophication has noticeably worsened in coastal ecosystems from the Baltic Sea, to the Adriatic Sea, to the Black Sea, to the Chesapeake Bay.

Diaz et al., 2008

While we are beginning to see the species specific effects of eutrophication and hypoxia, much is still unknown about the overall ecosystem impacts. So far, observations suggest that more energy is diverted into lower (microbial) trophic levels, at the loss of higher organisms in surrounding waters (depicted in the following figure).

Diaz et al., 2008

Nurseries and recruitment sites tend to suffer the most from the disproportionate energy allocations because hypoxia most often occurs in summer, which is an energy-intense season for predators. As might be predicted, regularly or seasonally hypoxic areas experience a decrease in secondary production by 1/3 to 1/2. Unfortunately, this pattern really is documented to occur near populated areas, and close to home, too: the Gulf of Mexico contains a dead zone (hypoxic area) reaching up to 15,000 square km born from the Mississippi's nutrient-rich waters. This is an unfortunate result of American farming and urbanization today, as seen in the following figure (Diaz et al., 2008).

coastalscience.noaa.gov

"Just Throw it Away"

Today, plastics easily find their way into the ocean after being dumped or spilled. Even if garbage is not deposited near a waterway, it can be picked up by rainwater and make its way into the ocean. The common misconception is that plastic does not degrade in the ocean, but accumulates in a gigantic garbage island. While these plastics do not biodegrade, they do photodegrade, producing small plastic particles and toxic chemicals in the process. Due to oceanic currents, these plastic particles accumulate in high concentrations in the five major ocean gyres shown below.

boatus.com

I recently had the pleasure of hearing Dr. Marcus Eriksen, a leading anti-plastics activist, speak of his many expeditions out into the worlds oceans. According to his website, 5gyres.org, the broken down plastic particles, including polucarbonate and polyestrene, never fully disappear, but become small enough to falsely attract marine organisms to ingest them. The United Nations Environmental Programme estimates that about 1 million seabirds and 100 thousand marine mammals die annually due to the presence of plastic in the ocean (Saido et al., 2009).
To make matters worse, these plastic particles actually attract toxins and contaminants within the water, such as PCBs (polychlorinated biphenyls) and DDT (dichlorodiphenyltrichloroethane). This means that as plastics circulate and degrade in ocean gyres, they are simultaneously acquiring more toxicity before (often) being ingested by a marine organism (5gyres.org).
Shockingly, 44% of seabirds, 22% of cetaceans, and all sea turtle species have been found with plastics in or around their bodies (as illustrated in the following images). This is ecologically damaging because ingestion or entanglement can lead to starvation, dehydration, and ultimately death (5gyres.org). 

questgarden.com

theplasticocean.blogspot.com

However, the risks of plastic in the ocean reach still further: what is plastic doing in our food web? If we eat animals who have eaten plastic, what impacts will those plastics and associated toxins have on us? The jury is still out as a great deal of research on this topic continues (5gyres.org). To me, it seems probable (and so very ironic) that the waste we so carelessly let drift out of sight and out of mind will ultimately come back to haunt human health.


From the Atmosphere to the Ocean: There is a Connection

Sources of greenhouse gas pollution, such as motor transportation and energy generation, do not only contribute to the greenhouse effect. Another major problem caused by carbon emissions is ocean acidification. In fact, according to a Scientific American article by Brian Bienowski, roughly 1/3 of carbon emissions from the burning of fossil fuels are now said to have been absorbed by the 'sponge-like' ocean water.
Ocean acidification happens when atmospheric CO2 is absorbed by the ocean, which reacts with water to form bicarbonate ions, effectively increasing the acidity. Acidic conditions in the ocean cause problems with the production of the calcium carbonate shells and skeletons of shellfish and corals. Consequently, the populations of such organisms decline, potentially causing a trophic cascade and a drastic change in species composition. This is because a huge number of fish species rely on coral reefs, and shellfish are important members of the marine food chain (Bienowski, 2013). For humans, coral is also an invaluable source of income from tourism. It might be considered even more valuable as a coastline protector from tsunamis for certain countries (Huffingtonpost.com, 2012).
Obviously both of these organisms are ecologically important, and a decline in their numbers could result in harm coming to the marine species that we rely on (shown in the following figure). 

thinkprogress.org

In his article, Bienowski claims that fish constitute an average of 6% of human protein. However, we know that seafood also traditionally composes the majority of certain diets around the world (Huffingtonpost.com, 2012). This means that any alteration to the ecologic systems that involve calciferous organisms might affect our diet. Negative affects are unfortunately already beginning to manifest in such forms as suffering oyster hatcheries, as acidity slows their growth (Huffingtonpost.com, 2012).

Oyster growth affected by ocean acidification, readthedirt.org

Hopefully the examples I have provided make it easy to see how the problems we cause can affect many other living organisms, and even come back to harm us. Eutrophication can cause severe ecological damage and decrease fish yields, economically impacting fishermen and limiting an important food source. Plastic pollution can kill marine organisms upon ingestion, and what doesn't kill them (likely) makes them toxic. This simultaneously decreases fish yields and endangers marine and human health. Ocean acidification causes disruptions in ecological systems and slows or stops the growth of economically important marine organisms. Unfortunately, the larger the human population, the further these situations can spiral out of control.

However, it is not all bad news, and there may be some improvement efforts for our oceans on the horizon. After a few more examples of why our population is the root of so many of our problems, I will explain ways in which humanity is beginning to be responsible for its actions.

Works cited:

Bienowski, Brian. U.S. Efforts on Ocean Acidification Needs to Focus on Human Impacts. Scientific American 11 Jan. 2013. Print. 27 May 2013.
Diaz, Robert J., et al. Spreading Dead Zones and Consequences for Marine Ecosystems. Science 321: 926-929.
Ocean Acidification is Climate Change's 'Equally Evil Twin,' NOAA Chief Says. Huffington Post 9 Jul. 2012. web. 27 May 2013.
Saido, Katsuhiko, et al. New Contamination Derived from Marine Debris Plastics. 238th ACS National Meeting, 2009.
What is the Problem. 5gyres.org 2013. web. 27 May 2013.