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 biogeochemical cycling, 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
by a mean of 4 µM and 0.2 µM, respectively, increasing dissolved
nitrogen: phosphorus ratios by a mean of 7 and slightly increasing
chlorophyll a by a mean of 1 µg L-1 in the upper 60 m of the
watercolumn, as well as increasing rates of nitrification within the
plume by a mean of 17 nmol L-1 day-1 and increasing δ13C and δ15N of
suspended particulate matter, an integrated measure of primary
production, by a mean of 1.3 ‰ and 1 ‰, respectively. We did not observe
a significant near plume effect on δ18O and δ15N of the dissolved
nitrate+nitrite, an indicator of nitrate+nitrite assimilation into the
biomass, instantaneous rates of primary production and respiration, or
dissolved oxygen concentration, suggesting any potential impact from
wastewater on these is moderated by other 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, may be insufficient to
document regional scale impacts of nutrients.