Runoff: How activities near and far from the ocean affect the ocean

I spent last week in the US Virgin Islands, collecting samples to reconstruct records of sediment runoff from the steep mountainsides that surround sparkling bays ringing the island of St. John. When it rains - which it does a fair bit, here - the rainwater breaks up the exposed rocks and soil on the mountainsides, and carries these particles downhill. As the rainwater enters the ocean, its speed slows and the sediment particles eventually settle out onto the seagrass beds and coral reefs and rocks lining the bays. The smaller particles stay in suspension longer and get carried farther out to sea.

River plumes - the visual and chemical evidence of how far river water protrudes into the ocean before it becomes too dilute to make out - can often be seen by eye from aerial photographs, particularly for large rivers with heavy sediment loads.

Sediment deposited into the marine environment can be stressful to marine life for a number of reasons - for instance larger particles can abrade organisms, small particles can interfere with gills, and sediment can block sunlight from reaching photosynthetic life. Sediment can also deliver pollutants to the water, bringing attached heavy metals or pesticides more swiftly into the ocean than perhaps otherwise.

Corals tend not to enjoy sediment settling on them, and spend energy cleaning sediment off either by producing mucus to trap it and have it carried away by the current, or by puffing up individual corallites with water to move the sediment particles off of the surface. The shape of coral colonies makes them either good at shedding sediment (for instance thin branching types or columns), or bad (for instance plate or gramophone-shaped colonies are poor at getting rid of sediment).

As you might expect, locations closer to shore tend to have more sediment runoff. Locations adjacent to developed land tend to have more as well: cutting down trees, leveling land to farm and build houses, making road cuts, and other forms of development tend to increase erosion. This happens because removing vegetation exposes soil and rock to rainfall; also, paved surfaces mean less rainfall permeates underground and more runs off over the surface, increasing erosion potential as well.

Even if you live a thousand miles from the nearest coastline, what you do can have an effect on the amount of sediment running off into the ocean. Essentially all rain that falls eventually makes it to the ocean. For instance, much of the land in the United States drains into the Gulf of Mexico through the Mississippi watershed.

So how can we reduce our sediment-runoff impact on the oceans? Vegetative buffers, terracing, cover crops, reducing impermeable surfaces, and engineering for sediment control can all help.

In St. John, specific efforts have been undertaken to reduce sediment runoff. For instance, some unpaved roads have had drainage control systems implemented, and native vegetation effective at erosion control is being used to mitigate erosion associated with development. With evidence that reducing local stressors like runoff may help corals withstand the ravages of climate change, these kinds of projects may be key to the survival of coral communities into the future.

References

Syvitski, J. P., Vörösmarty, C. J., Kettner, A. J., & Green, P. (2005). Impact of humans on the flux of terrestrial sediment to the global coastal ocean. Science, 308(5720), 376-380.

Rogers, C. S. (1990). Responses of coral reefs and reef organisms to sedimentation. Marine Ecology Progress Series, 62(1), 185-202.

Ramos-Scharrón, C. E. (2012). Effectiveness of drainage improvements in reducing sediment production rates from an unpaved road. Journal of Soil and Water Conservation, 67(2), 87-100.