United States
Environmental Protection
Agency
Environmental Research
Laboratory
Narragansett Rl 02882
Research and Development
EPA/600/S3-87/006 July 1987
&EPA Project Summary
Response of Benthic
Ecosystems to Deep Ocean
Sewage Outfalls in Hawaii: A
Nutrient Cycling Approach to
Biological Impact
Assessment and Monitoring
Stephen V. Smith and Steven J. Dollar
The full report describes the benthic
nutrient budgets, metabolism, and
community structure observed around
two deep ocean sewage outfalls off
Oahu, Hawaii. The overall effect of
effluent on benthic and pelagic ecosys-
tems off Oahu is insignificant—prim-
arily as a result of effective dispersal
and dilution. The results also indicate
that a community function approach
may, in some situations, be more
efficient and effective than a commun-
ity structure approach for ecosystem
assessment and monitoring.
This Project Summary was devel-
oped by EPA's Environmental Re-
search Laboratory, Narragansett. Rl, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The intent of the research presented
in the full report is to test the applicability
of a new methodology to assess the
response of the benthic community off
Oahu, Hawaii, to sewage effluent dis-
charged from two deep-ocean outfalls.
A simple material flux approach was
employed to measure delivery of sewage
material to the sediment-water interface,
storage of sewage material in the sed-
iment, and metabolic recycling of effluent
nutrients. The potential utility of the flux
model is that the fate and effect of the
sewage can be quantified in terms of total
ecosystem function. For a material flux
approach to constitute an improvement
in the effectiveness and efficiency of
practical ecosystem assessment and
monitoring, it should provide more useful
information than traditional monitoring
strategies. In order to make such a
comparison, a "standard" monitoring
program based on identifying changes in
infaunal community structure was con-
ducted in conjunction with the benthic
flux studies.
The Study Sites—Oahu Deep
Ocean Sewage Outfalls
The Sand Island Wastewater Treat-
ment facility currently treats domestic
waste from residential areas, the Hono-
lulu business district and the Waikiki
resort area, and the concentrations of all
priority pollutants fall below Federally
acceptable standards. It provides
advanced primary treatment for an
average flow of 30.5 x 104 m3 day"1 with
an average suspended solids load of 40
mg 1"1. The wastewater treatment
process consists of pre-screening, dis-
solved air flotation, clarification and final
screening.
The Hohouliuli (Barbers Point) waste-
water facility and deep ocean outfall
became operational in 1981. The service
area for this system extends from Halawa
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to Barbers Point and consists primarily
of residential communities with small
local business centers. The two major
municipal facilities that discharge into
the Honouliuli system (Pearl Harbor and
Waipahu) previously discharged directly
into Pearl Harbor. Discharge from the
Honouliuli outfall is approximately 6.5 x
104 m3 day"1 with an average suspended
solids concentration of 120 mg 1~1
During the period of field study, the
treatment plant facility was not operat-
ing. Influent was coarse-screened prior
to discharge through the ocean outfall.
Therefore, the effluent was essentially
untreated raw sewage.
While the Sand Island Outfall has a
definite seasonal variability, the hydrau-
lic loading at Barbers Point is fairly
consistent throughout the year.
The marine environments in the vic-
inity of both outfalls and the control site
consist of a low-relief calcium carbonate
sand bottom. Very few macro-benthic
invertebrates or demersal and pelagic
fish were observed in the outfall or
control environments. The outfall pipes
and armor rock provide areas of
increased habitat complexity in other-
wise barren areas. As a result, there are
outfall-associated fauna! communities
with relatively high diversity and
biomass.
The control station is located off
Waikiki at a depth of 70 m in the region
of Mamala Bay judged to be subjected
to the least influence from discharge of
both the sewage outfalls and Pearl
Harbor. Bottom composition and current
flow at the control site appeared similar
to those of the outfall sites.
Conclusions
A set of relatively "standard theory"
biogeochemical concepts and methodol-
ogies designed to quantify material
fluxes of carbon, nitrogen and phospho-
rus was used to characterize functional
ecosystem response to the Sand Island
and Barbers Point (Honouliuli) deep-
water sewage outfalls. Direct measure-
ments of particulate flux indicate that
only about 1% of the material reaches
the sediment water interface; the
remaining 99% is dispersed by currents.
Very little of the material that reaches
the interface is incorporated into the
sediment column. Vertical profiles of
particulate organics show no concentra-
tion gradients in the upper 4 cm of
sediment and only slight differences in
concentrations between control and
outfall sites. The organic material in the
sediment appears to be the most recal-
citrant portion of organic fallout. Lower
C:N and C:P ratios of the sediment
organics compared to particulate fallout
indicate that long-term abiotic processes
of adsorption and humification may
cause sequestering of N and P in the
sediment column.
Ratios of stable isotopes of nitrogen
indicate that near the outfall structures,
there is a substantial sewage component
to the surface sediment organic material.
It also appears that naturally occurring
organic material may originate predom-
inantly from recycled benthic detritus
emanating from the nearshore region,
rather than pelagic (plankton) fallout.
In-situ benthic fluxes indicate that
essentially all of the sewage-related
particulate material that reaches the
sediment-water interface is rapidly
recycled to dissolved forms. This con-
trasts with the control area, where only
about half of the particulate fallout is
recycled. The difference appears to result
from the highly labile character of the
sewage particulates as compared to the
relatively refractory "old" material
fueling natural benthic processes.
Benthic fluxes are apparently caused
primarily by aerobic metabolism. Ratios
of N:P flux indicate that both nutrients
are released back to the water column
at similar rates in about the same ratio
as sewage effluent
Although the magnitude of fluxes
varies with site and time, metabolic
processes are qualitatively consistent.
Stoichiometry of C>2 uptake and dissolved
nitrogen release indicates that respira-
tion and nutrient regeneration are not
closely coupled.
Pore-water profiles of NH4+, NOa" +
NO2+, PO*3', have concentration gra-
dients in the top 1 to 2 cm of the sediment
column that produce similar ionic diffu-
sive fluxes across the interface at both
outfall and control sites. These fluxes do
not mirror the variations in measured net
benthic flux that appear to be functions
of the outfall influence. Ratios of net to
diffusive flux greater than unity, as well
as the lack of bioturbation and wave/
current stirring, suggest that the changes
in net flux are caused by rapid remin-
eralization of effluent material at the
sediment surface. Ratios of net to
diffusive flux of near unity from the
control sites indicate that most of the
diagenetic activity is occurring within the
sediment column when there is no
anthropogenic input.
Most in-situ measured dissolved N flux |
is the result of NH«+ production from "
deamination and ammonification. Nitrate
and nitrite flux is low and does not follow
the major trends of other constituents,
possibly because the rates of nitrification
are too slow to reach steady-state within
the experimental time.
Diffusive flux of P043" is greatly
affected by sediment adsorption, causing
net uptake at the control sites. However,
surface metabolism appears to be rela-
tively unaffected by this abiotic process
and the effect is masked at the outfall
sites.
At Barbers Point, benthic fluxes of
dissolved nutrients in the vicinity of the
diffuser are consistently higher than in
control areas during both years of study,
1984 and 1985. Organic floes of sewage
origin were also observed on the sed-
iment surface within 50 m of the Barbers
Point diffuser.
At Sand Island, no organic floes were
observed. Benthic flux is greater than the
control at Sand Island only during the
sampling, coinciding with pulse inputs of
elevated dissolved BOD from seasonal
discharge of a pineapple cannery. The
increased BOD apparently triggers the
growth of microbial mats which fragment
to provide a particulate nutrient source
to the benthos. Therefore, only when raw
sewage particles or secondary sources
of particulate material (mats) reach the
sediment-water interface is there an
influence on benthic community func-
tion. In the present study this influence
to the benthos is limited to within the
zone of initial dilution (ZID).
Analysis of sediment macrofauna and
meiofauna at Sand Island and Barbers
Point indicates that beyond the ZID, no
significant changes occur m community
structure compared to the control. Within
the ZID at Sand Island there are statis-
tically significant decreases in diversity,
number of species, and number of
individuals but no significant changes in
biomass. Within the ZID at Barbers Point,
there is no significant change in diversity,
species number, or number of individ-
uals, but biomass is increased. These
patterns do not typify ecosystem
response to nutrient subsidy or eutrophi-
cation (i.e. decreased diversity and
number of species, increased biomass
and number of individuals).
While no clear-cut relationship exists
between community structure and efflu-
ent input, a possible cause for the
depressed community structure near the
Sand Island diffuser is the absence of
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I filter feeders, possibly owing to periodic
pulses of bacterial mats. If this is the
case, the inter-pulse interval may be too
short to allow recolonization of the filter
feeders.
Macrofauna (> 0.5 mm) may serve to
accurately characterize the benthic
fauna at the Oahu outfall sites as long
as the sample size and number of
replicates is adequate.
The Oahu outfalls are ideal for com-
paring community structure and function
as environmental management tools,
because no obvious changes occur in
either the physical or the macrofaunal
environments. If large aggregations of
indicator species, such as Capitella
capitella, were present in the vicinity of
the Oahu outfalls, community structure
analyses would have been adequate to
recognize biological impact. On the other
hand, if organic deposition near the
outfalls caused the formation of an
anaerobic sediment layer, such a change
in the benthic community function would
serve to identify ecosystem impact.
The nutrient cycling approach, utilizing
direct measures of fluxes into and out
of the benthic boundary zone, is verified
as a viable and sensitive approach to
quantifying the potential for environmen-
tal stress at relatively low levels of
anthropogenic influence. Although the
influence also results in discernable
alterations to infaunal community struc-
ture, what causes the alterations and
what they indicate about the effect of the
sewage discharge on ecosystems remain
unclear.
From an environmental management
standpoint, the effects of the sewage
outfalls upon benthic and pelagic eco-
systems are insignificant. However, the
augmentation to benthic flux is substan-
tial enough to change the characteristics
of metabolic function of the benthic
boundary zone from that typical of the
deep sea to that of estuaries or coastal
seas.
In addition, the benthic function anal-
ysis indicates that even in open ocean
outfalls, primary treatment of effluent is
preferred to raw effluent discharge. Even
with primary treatment highly labile
dissolved organics may alter environ-
mental quality.
At present, the rationale of biological
assessment and monitoring is based
solely on the identifying changes in
specific community parameters within
specific areas of the environment relative
to the input source; hence, the emphasis
on fauna inventories. However, broaden-
ing the view of assessment and moni-
toring to include total ecosystem
response, as well as the input functions
that cause the response, appears to yield
much more potentially useful informa-
tion for evaluating the performance of
outfall discharge systems.
Because the methods are available to
determine subtle changes in community
function, it is possible to identify ecosys-
tem changes at levels of stress below the
threshold necessary for changes in
community structure to become discern-
able. Hence, analysis of community
function may be a more practical method
of assessment and monitoring.
The levels of technology and capital
investment required for metabolic flux
analyses are considerably more
advanced than those required for com-
munity structure analyses. In the case
of the Oahu outfalls man hours were
approximately equal for the two
approaches, but results of community
structure analyses were rather ambigu-
ous and did relatively little to address the
cause-effect relationships of sewage
effluent on a small segment of the marine
ecosystem. Community function anal-
yses, on the other hand, resulted in a
clear descriptive and statistical picture of
impacts on the entire benthic
community.
In addition, following the initial invest-
ment (and provided submersible capabil-
ities are available) the equipment and
analytical facilities required to conduct
future "community function" programs
are relatively basic and inexpensive.
Because the data generated by functional
analyses are available almost instan-
taneously, regardless of the number of
replicates, there is no delay in seeing
results, as is often the case when large
numbers of benthic samples are col-
lected. Therefore, for long-term repetitive
monitoring programs, the community
function approach may be a cost-
effective alternative to the community
structure approach.
Recommendations
The results of this study indicate that
simple biogeochemical balances of
material fluxes from the benthos in the
vicinity of domestic sewage discharges
provides an effective and efficient
method of characterizing environmental
impact. However, although the func-
tional approach has been demonstrated
to be useful for characterizing small
changes under conditions of marginal
stress, it would be valuable to apply the
techniques to discharge situations with
substantial environmental degradation.
While deployment of an assessment
and monitoring program based on ben-
thic community function is best handled
at deep ocean sites with the use of
research submersibles, all of the neces-
sary techniques can be conducted from
surface-operated equipment. However,
the availability of a research submersible
improves the capabilities of the program,
and appears to be one of the more
beneficial uses of undersea technology.
It is recommended that when submers-
ible time is available for assessment and
monitoring programs, a share of the time
be designated to perform some form of
functional assessment of ecosystem
response.
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Stephen V. Smith and Steven J. Dollar are with University of Hawaii, Honolulu,
HI 96822.
Steven Ferraro is the EPA Project Officer (see below).
The complete report, entitled "Response of Benthic Ecosystems to Deep Ocean
Sewage Outfalls in Hawaii: A Nutrient Cycling Approach to Biological Impact
Assessment and Monitoring," (Order No. PB 87-165 205/AS; Cost: $18.95,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Research Laboratory
U.S. Environmental Protection Agency
Narragansett. Rl 02882
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-87/006
IL
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