Annual and seasonal dissolved inorganic nutrient budgets for Humboldt Bay with implications for wastewater dischargers
Graduation Date
2015
Document Type
Thesis
Program
Other
Program
Thesis (M.S.)--Humboldt State University, Environmental Systems: Environmental Resource Engineering, 2015
Committee Chair Name
Brad A. Finney
Committee Chair Affiliation
HSU Faculty or Staff
Keywords
Silicon, Eutrophication, Nutrients, Humboldt Bay, Nitrogen, Nutrient budget, Phosphorus, Nutrient uptake, Nutrient loading, Water quality, Humboldt State University -- Theses -- Environmental Resources Engineering
Abstract
Dissolved inorganic nutrient loading and uptake have been estimated for each of the four major compartments of Humboldt Bay for the two major seasons characterized by ocean upwelling (April through September) and watershed runoff (October through March). Dissolved inorganic nutrient loading estimations include dissolved inorganic nitrogen (DIN) as ammonium-N, nitrate-N, and nitrite-N, dissolved inorganic phosphorus (DIP) as phosphate-P, and dissolved inorganic silicon (DSi) as silicate-Si. DIN and DIP uptake estimations include phytoplankton, macroalgae, and eelgrass production, intertidal sediment flux, and denitrification. A water budget including tidal flows, watershed runoff, wastewater discharge, and direct precipitation on the Bay is also included as a means for determining mass inputs from various sources using concentration data. Humboldt Bay is a nitrogen limited system exhibiting stoichiometric N:P ratios below the 16:1 Redfield ratio. N:P ratios decrease significantly inside Arcata Bay (the inner-most compartment of Humboldt Bay) compared with water near the Bay entrance during the upwelling season, indicating that denitrification is a major contributor to nitrogen removal from the system during these periods. This also suggests that Arcata Bay is more nitrogen limited than nearshore waters, as denitrification removes N and not P from the system. Annual estimates of denitrification in Humboldt Bay using areal denitrification rates from a nearby tidal estuary indicate that denitrification may be over five-times greater than the total wastewater DIN discharge, 768 Mg N/yr and 149 Mg N/yr, respectively. Estimates of phytoplankton, macroalgae, and eelgrass production in the Bay are also greater than wastewater DIN discharge loads. Freshwater DIN and DIP loads to Humboldt Bay are minor in comparison with estimates of nearshore nutrient loading. Average annual DIN and DIP loading to Humboldt Bay from nearshore waters is approximately 14,363 Mg N/yr and 2,653 Mg P/yr, respectively, with only 1.7% of the total nearshore load, 239 Mg N/yr and 44 Mg P/yr, respectively, estimated to be directly loaded to Arcata Bay. DIN inputs to Arcata Bay from freshwater sources including wastewater and watershed runoff contribute 40 Mg N/yr and 51 Mg N/yr, respectively, or 17% and 21% of the estimated load from nearshore waters, respectively. DIP inputs to Arcata Bay from wastewater and watershed discharges contribute 13 Mg P/yr and 6 Mg P/yr, respectively, or 21% and 10% of the nearshore load, respectively. During the upwelling and runoff seasons, the Arcata wastewater treatment facility (AWTF) DIN discharge to Arcata Bay makes up 5% and 18% of the total load to the Bay, respectively, and 16% and 25% of the DIP load, respectively. Eutrophication potential in Humboldt Bay increases during the productive upwelling season as biological uptake of DIN and DIP increase by an estimated 250% and 415%, respectively, and nearshore loading increases by 20% and 14%, respectively. Watershed runoff DIN and DIP loads decrease during the upwelling season by an estimated 74%, and wastewater loads decrease by an estimated 32% and 20%, respectively. Decreased DIN and DIP discharge from wastewater and watershed sources during the productive upwelling season suggest that anthropogenic nutrient impacts to potential eutrophication in Humboldt Bay are minor in comparison to nearshore influences.
Recommended Citation
Swanson, Charles R., "Annual and seasonal dissolved inorganic nutrient budgets for Humboldt Bay with implications for wastewater dischargers" (2015). Cal Poly Humboldt theses and projects. 1279.
https://digitalcommons.humboldt.edu/etd/1279
https://scholarworks.calstate.edu/concern/theses/zw12z779k