Quantifying the Effect of Aquaculture on Nutrient Flux at the Sediment-Water Interface in Maine
Project Description
Sediment processing of aquaculture biodeposits is a key factor in determining the carrying capacity of aquaculture operations. Sediment biogeochemical processes lead to the mineralization of biodeposits high in carbon and nitrogen content, and increase the flux of inorganic nutrients to the water column. The effect of aquaculture operations on water quality regarding sediment nutrients (carbon, nitrogen, phosphorus) and flux within the three bioregions, with an emphasis on the Damariscotta, is being determined. This knowledge will locate areas best suited for raising marine species, and enable the safe increase of aquaculture operations. The sediment nutrient budget and benthic impact of aquaculture facilities in the upper Damariscotta River are being evaluated.
Researchers have investigated nitrogen cycling and flux in the upper Damariscotta River under ambient conditions using flux chambers. They will investigate the effect of benthic diatom mats prevalent in this reach of the river. Benthic diatom mats are hypothesized to greatly influence nitrogen cycling.
Results and Accomplishments
Flux chamber experiments continued in 2017. These experiments involved the collection of intact sediment cores (via a vessel or diving) by their incubation under ambient conditions to measure the flux of nitrogen species at the sediment-water interface. The 2017 sampling involved intact sediment cores from the Norumbega and Mook’s oyster farms in the upper Damariscotta River and Long Cove. The latter site was relatively unaffected by the aquaculture operations and chosen as a reference point in the river. Flux chamber experiments were conducted on the same day at ambient temperature to monitor fluxes of nitrogen species (ammonium, nitrate and nitrogen gas). Sediment oxygen demand and closed-cell pH were measured.
Sediments associated with the upstream oyster farms had a higher oxygen demand due to higher concentrations of organic matter than the downstream sediments. Similar to the 2016 findings, sediment at all sites, under all conditions, released insignificant amounts of nitrate. Project personnel discovered oyster farm sediments release ammonium to the water column, whereas sediments from the downstream location are less affected by the farms take-up of ammonium from the water column (i.e., negative ammonium flux).
Nitrogen gas released due to the reduction of nitrate (denitrification) was measured using the membrane inlet mass spectrometer (MIMS). These results were inconclusive, as they did not show significant differences between the locations. Denitrification may be an important process in sediment nitrogen cycling, but the findings do not suggest whether the low nitrate concentrations are due to denitrification at the sediment-water interface or the uptake by the benthic diatoms that are observed in dense mats close to the shallow upstream sites near the oyster farms. Future studies will consider these mats and their role in nitrogen storage and potential release.
Sediment at all sites and under all conditions released carbon dioxide, indicating sediments are sources of this gas and not a sink. As such, the incremental increase in the atmospheric carbon dioxide does not affect the sediment pH; the sediment pH is controlled by the biogeochemical processes initiated by the mineralization of the organic matter taking place close to the sediment-water interface.
Summary of Data Being Collected
| Data | Type | Quantity | Location |
| Dissolved oxygen, temperature and photon flux | Chemical | >150 | Darling Marine Center, UMaine |
| Nitrogen (nitrate and ammonium) and phosphorus species | Chemical | >150 | Collected at Darling Marine Center, UMaine; analyzed at Soils Analytical Laboratory, UMaine |
| Nitrogen gas; nitrogen:argon ratio | Chemical | >50 | Collected at Darling Marine Center, UMaine; analyzed at the Horn Point Marine Laboratory, University of Maryland |
| Greenhouse gases (carbon dioxide and methane) | Chemical | >50 | Collected at Darling Marine Center, UMaine; analyzed at the Water Quality Analysis Laboratory, University of New Hampshire |
| Sediment chlorophyll a content | Chemical | 20 | Environmental Chemistry Laboratory, UMaine |