Nitrogen from Marine Sediments
When algae in the surface waters of Puget Sound take up inorganic nitrogen, they convert this nitrogen into organic forms. The algae eventually die and decompose, settling onto the Puget Sound’s floor, supplying organic nitrogen to marine sediments. This organic nitrogen is further degraded by microbes in the sediment, back into inorganic nitrogen which is then returned to the water column. This exchange of organic and inorganic nitrogen across the sediment-water interface is referred to as the benthic sediment flux. This process typically results in a net supply of inorganic nitrogen to the water column from the sediments.
Benthic fluxes of nitrogen were measured in South Puget Sound as part of the field data collection effort for the South Puget Sound Dissolved Oxygen Study. Measurements were made using benthic flux chambers deployed in the late summer/early fall of 2007 at depths of 5, 15, and 25 meters within the Case, Carr, Budd and Eld Inlets in South Puget Sound. These measurements indicated that the sediment was a source of DIN to the water column. Individual rates varied from 0 to 130 mg-N/m2-d, with an overall grand mean of 52 mg-N/m2-d (Roberts et al., 2008). Slightly higher values were measured in a study by King County in Quartermaster Harbor, which found benthic flux rates of 0 to 150 mg-N/m2-d (DeGasperi, 2012).
Using the grand mean DIN flux of 0.052 g-N/m2-d measured in the South Puget Sound Dissolved Oxygen Study, Roberts et al., (2008) then calculated the following DIN loads from the sediment to different South Sound waterbodies:
*Values rounded to the nearest 100 kg/d (Source: Roberts et al., 2008)
The total marine sediment DIN load of 21,723 kg/d to South Sound from benthic fluxes is about six times greater than the total average summer DIN load of 3,690 kg/d from all rivers, streams and WWTPs that discharge to South Puget Sound. While this is likely a high estimate for the deeper portions of South Puget Sound, the estimates suggest that sediment processes play an important role in nitrogen cycling, and particularly in shallow waters.
The benthic flux DIN loads in the table above were measured in late-summer conditions, and may not be representative of year-round conditions. They also represent inlets in South Sound, which are shallower (< 25 m deep) and often warmer than the rest of Puget Sound. Benthic flux rates are expected to be lower during the winter and in deeper, cooler waters, due to reduced biological activity.
The USGS, under a grant from Ecology, performed an extensive literature review to summarize existing measurements of benthic flux of inorganic nitrogen collected in different parts of Puget Sound. Fluxes were found to be influenced by a number of factors, including measurement methods, location in Puget Sound, depth of measurements, bottom temperature, and season. Sediment composition and organic matter are also important factors, but the effect remains unknown.
Overall patterns/results from this study are summarized below:
Below is a table with summary statistics and a box plot summarizing the flux measurements found by the USGS literature review.
Summary statistics of benthic nitrogen fluxes in Puget Sound (Source: adapted from Table 7 and Figure 8 from Sheibley and Paulson, 2014)
By applying the median benthic flux values from the USGS study to the entire Puget Sound, we developed an initial inorganic nitrogen load estimate of 40,200 kg/d to Puget Sound from marine sediments, as illustrated below. This value is larger than the total annual average dissolved inorganic nitrogen load from wastewater treatment plants (32,200 kg/d) and rivers (22,600 kg/d).
However, this estimate is based on a single benthic flux value applied to the entire Puget Sound area – in reality, flux values will vary spatially across Puget Sound based on a number of variables, as described earlier, including depth, temperature, sediment composition, etc.
Given the potentially large influence of nitrogen from marine sediments to Puget Sound, Ecology is currently planning on adding the simulation of nutrient fluxes (a process also known as sediment diagenesis), to an existing water quality model of Puget Sound. A quality assurance project plan is currently being finalized to start this work, with results anticipated by the end of 2015.
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