Sources of Nitrogen Utilized by Macroalgae: How Clean is your Kelp?
*work in collaboration with industry partner, Todd Jagoutz
Project Description
Nutrient pollution can have substantial impacts on estuaries and bays within the Gulf of Maine. High nitrogen or phosphorous inputs can trigger large, unwanted microalgal blooms that produce low-oxygen zones. This kills fish and crustaceans, resulting in massive loss of eelgrass beds. The water also develops an unsightly color and odor. Tourism, recreation, commercial fishing, aquaculture, nearby property values, and human health are affected by these blooms.
Numerous studies have been conducted to evaluate the efficacy of strategically growing macroalgae in areas experiencing nutrient pollution. This remedial approach leverages the macroalgae’s natural need for nitrogen and phosphorous. When the nutrients are assimilated by the macroalgae, they are no longer available to the microalgae that produce nuisance blooms. Applied as a resource management practice, this is often referred to as bioremediation or bioextraction. Researchers need to have a better understanding of preferential uptake and utilization of different nitrogen sources by the farmed kelp.
This project is conducted in partnership with kelp farmers located in Saco, Casco, and Penobscot Bays. Nitrate, nitrite, and total Kjeldahl nitrogen (TKN) levels in nearshore marine waters are compared to the total nitrogen and nitrate reductase activity in the kelp tissue. This comparison can shed light on the quantity and rate of nitrogen assimilation occurring in the kelp. Measuring TKN (organic N + ammonia) is important because this is the regulatory criterion used by the Clean Water Act. The degree to which the farmed kelp is assimilating nitrogen from anthropogenic sources can be determined by measuring the nitrogen isotope value, or δ15N, in the kelp tissue. Discharge from storm sewers and over-ground fertilizer runoff are all sources of anthropogenic nitrogen has a δ15N value distinct from nitrogen provided by atmospheric deposition or upwelling.
Nitrogen observations are paired with morphology measurements to determine if there is a relationship between: a) nitrogen in the water and nitrogen in the kelp, b) nitrogen in the kelp and its morphology, c) nitrogen in the kelp and seasonal factors like temperature, and d) location of the kelp and amount of nitrogen from anthropogenic sources in the kelp.
The outcomes of this project may have implications for: urbanization, harvesting, and investment activities.
Results and Accomplishments
One of the goals of this project is to measure δ15N in macroalgae to identify sources of nitrogen utilized by different species macroalgae. By measuring δ15N in both kelp and rockweed, personnel can better understand the seasonality of anthropogenic sources of nitrogen and determine whether there is any risk of farmed kelp receiving influence by effluent. The availability of dissolved nitrogen limits growth of primary producers such as macroalgae. Sources of nitrogen (ammonia, nitrate, nitrite) can either be natural or anthropogenic (e.g. sewage effluent). Understanding the source of nitrogen and how it is used by primary producers is important for management of waste water and Maine marine crops such as kelp (Laminaria spp).
This objective was met by measuring and comparing total N and the isotopic ratio of N in farmed kelp and wild rockweed, namely Fucus spp and Ascophyllum spp, across seasons. Nitrogen (N) exists as one of two stable isotopes, 14N and 15N. Because 15N (0.4%) is much rarer than 14N (99.6%), the ratio between the two can be used as an indicator of the source of N that organisms incorporate into their tissues (Fry 2007). Sewage effluent has proportionally more 15N than 14N and a higher N isotope ratio than oceanic water (Savage 2005). This ratio of the two N isotopes is conventionally expressed as δ15N and can be used to determine the relative impacts of sewage effluent on macroalgal growth from macroalgal tissue samples.
The outcomes of this project suggest little variability in δ15N across species or locations within Saco Bay. There does seem to be some difference in N signature across different parts of individual alga specimens: stipe, mid-blade, tip of blade. Further work is being done with project partners at University of Southern Maine to look at variability across bays, including Casco Bay.
Project personnel hypothesize that anthropogenic activities on land and in the ocean may directly affect rates, quantities, and seasonal patterns of nitrogen available to kelp farmed in Casco, Saco, and Penobscot bays. Local marine dynamics and the species-specific nitrogen assimilation capacity of kelp may allow seaweed farming and harvesting to play an active role in nitrogen management and trading schemes. The degree to which kelp farming can be incorporated into nutrient trading programs may depend on land-based nitrogen inputs to coastal waters.
Summary of Data Being Collected
| Data | Type | Quantity | Location |
| Stable isotope C and N values | Kelp and rockweed | Collected kelp every 2 weeks through summer of 2016. Collected rockweed year-round, May 2016-August 2017 | Saco Bay |