A Living, Breathing Nutrient-Recycling Bin
As the ship sails from Vineyard Haven to New Bedford, a city built on the whaling trade, the Morgan carries two Voyagers whose roots are equally entrenched in the industry. Matthew Bullard is the fourth-great grandson of Charles W. Morgan himself, and Peter Whittemore is author Herman Melville’s great-great grandson. While it is likely that neither ancestor would recognize the scientific procedures conducted on the voyage, they would surely appreciate the importance of the information that this science will produce.
Within just a few generations, much has changed in the world. Part of the purpose of this twenty-first century voyage is to quantify any change in the waters once roamed by the Morgan and her counterparts. To do so, the Voyagers take advantage of modern technology to glean as much data from their water samples as possible. From a rubber bucket filled at every hour along the voyage track, water is stored in a carefully labelled bottle to be frozen at the next port stop for later analysis. Students with the Williams-Mystic Maritime Studies Program will spend next fall semester measuring the nutrient concentrations in the locations sampled during the 38th voyage, focusing largely on nitrates.
Nitrates are commonly brought into coastal waters via runoff or upwelling. Phytoplankton encounter these nutrients in the upper level of the ocean, known as the photic zone. Here, these plant organisms utilize the available sunlight and nutrients, such as nitrates, in order to photosynthesize and grow. When zooplankton enter the photic zone to feed on the phytoplankton, they release nitrate-rich fecal pellets that sink to the seafloor. The same process takes place as larger vertebrates feed on the zooplankton. In this way, waste transports nitrogen downward, away from the surface waters.
Though they often feed on zooplankton and other small fish, marine mammals are also critical to photosynthesis in the ocean. A Voyager on the first Provincetown leg, UVM biologist Joe Roman, and his colleague recently discovered that these larger organisms actually serve as another nitrogen source at the surface (Roman & McCarthy, 2010). Unlike most marine vertebrates, whales need to return to the surface in order to breathe and excrete waste. In doing so, they supply nitrates back to the photic zone. This form of nutrient-recycling is known as a “whale pump,” and it helps maintain photosynthesis in areas highly frequented by whales.
Nutrient analysis from the 38th Voyage suggests that nitrate levels were only slightly higher than expected for this time of year, crucial information in terms of determining water quality and ecosystem health. While nitrates are a key limiting nutrient for phytoplankton growth in the Gulf of Maine, high levels of nitrates and other nutrients can actually produce a destructive phenomenon known as eutrophication. This occurs when an excess of nitrates enters the ecosystem, increasing the size of algal blooms. Algal blooms deplete the oxygen in the water necessary for other organisms to survive, and some algae species are toxic to both marine animals and humans.
Unfortunately, human civilization contributes substantially to eutrophication. Nutrients can reach the coast naturally though upwelling from the deep ocean, but many nutrients come from people. When nitrogen fertilizers are used to enrich soils, nitrogen-filled agricultural runoff drains with wastewater from sewage treatment facilities into coastal waters. As the data from the 38th Voyage suggests, modern-day scientists are trying to better understand the role of nutrients in a healthy ecosystem in which everything, from tiny phytoplankton to the majestic whale, can thrive.