EPA's Vessel General Permit and Small Vessel General

Response Analysis
The previous Exposure Analysis section reviewed the discharges in terms of their volume and
constituent mixtures, then described EPAs harbor modeling methodology and results. This
section summarizes the modeled harbor discharge constituent concentrations for each pollutant
class and the relative importance of discharge sources before reviewing the responses of listed,
or surrogate, species to exposures to these pollutants.
Nutrients
In their Biological Evaluation EPA acknowledged that Nutrient pollution is one of the leading
causes of water quality impairment in the nation, primarily because the quantity of nutrients
reaching the nations waters has dramatically escalated over the past 50 years (USEPA 2009).
Nutrient loadings in the form of nitrogen and phosphorus to waterbodies impact water quality by
stimulating plant and algae growth which subsequently may result in depletion of dissolved
oxygen, degradation of habitat, development of harmful algal blooms, impairment of the
waterbody’s designated use, and impairment of drinking water sources. In general, nitrogen is
most often the limiting nutrient in estuarine waters, and phosphorus is more often limiting in
freshwater systems. This means that the growth of phytoplankton is substantially controlled by
the concentration and availability of phosphorus in freshwater systems. Increased phosphorus
concentrations can lead to changes in composition of flora and fauna present, increased
eutrophication of a water body, rates of ecosystem functioning, nutrient uptake, recycling rates of
the ecosystem, and decomposition rates (WERF 2010). Determining risk to aquatic life from
excess nutrients (e.g.,eutrophication) is complicated because nitrogen and phosphorus are
essential for primary production in aquatic ecosystems, and over-enrichment problems involve
multiple interrelated variables. The most visible symptom of eutrophication is the excessive
algal growth that reduces water clarity. Eutrophication can also significantly affect
phytoplankton community structure resulting in a greater abundance of less desirable taxa such
as blue-green algae. These changes in the phytoplankton community can have cascading effects
on higher trophic levels and the eventual transfer of organic carbon from the primary producers
to less desired species – for example, the replacement of seagrasses with less desirable
vegetation types (WERF 2010).
Nutrients in Vessel Discharges
EPA reported that nutrients, particularly ammonia nitrogen and total phosphorus, were
constituents commonly found in bilgewater, deck washdown, fish hold and fish hold cleaning
effluent, and graywater. Based on studies of pollutants in vessel discharges EPA reported that
average ammonia nitrogen concentrations were above their screening level benchmark of 1.2
mg/L in both fish hold effluent and fish hold cleaning effluent (USEPA 2010b) (see Appendix D
of the BE for more information regarding screening level benchmarks). Protein, free amino
acids, and nucleotides from fish and fish by-products are all potential sources of nitrogen in the
fish hold effluent and fish hold cleaning effluent. EPA also reported that ammonia nitrogen
concentrations can also be above screening level benchmarks in graywater. EPA found the
average ammonia nitrogen concentration from all graywater sources on cruise ships was 2.1
277