Influence of Anthropogenic Nutrient Inputs on Rates of Coastal Ocean
Nitrogen and Carbon Cycling in the Southern California Bight, USA
Abstract
Coastal nitrogen (N) enrichment is a global environmental problem that
can influence acidification, deoxygenation, and subsequent habitat loss
in ways that can be synergistic with global climate change impacts. In
the Southern California Bight, an eastern boundary upwelling system,
modeling of wastewater discharged through ocean outfalls has shown that
it effectively doubles N loading to urban coastal waters. However,
effects of wastewater outfalls on biogeochemical rates of primary
production and respiration, key processes through which coastal
acidification and deoxygenation are manifested, have not been directly
linked to observed trends in ambient chlorophyll a, oxygen and pH. In
this paper, we compare observations of nutrient concentrations and
forms, as well as rates of nitrification, primary production, and
respiration, in areas within treated wastewater effluent plumes compared
to areas spatially distant from ocean outfalls where we expected minimum
influence of the plume. We document that wastewater nutrient inputs have
an immediate, local effect on nutrient stoichiometry, elevating ammonium
and nitrite concentrations and increasing dissolved nitrogen: phosphorus
ratios, as well as increasing rates of nitrification within the plume.
We did not observe a near plume effect on nitrate assimilation into the
biomass, primary production, chlorophyll a, respiration, or dissolved
oxygen concentration, suggesting any potential impact from wastewater on
these processes is moderated by offshore factors, notably mixing of
water masses. These results indicate that a “reference-area” approach,
wherein stations within or near the zone of initial dilution (ZID) from
the wastewater outfall are compared to stations farther afield
(reference areas) to assess contaminant impacts, is insufficient to
document regional scale impacts of nutrients. Understanding of the
complex interactions between local, regional, and global drivers on
coastal eutrophication requires coupled observational-numerical modeling
approaches where numerical models are carefully validated with observed
state and rate data to develop effective, evidence-based solutions to
coastal eutrophication.