United States
Environmental Protection
Chesapeake Bay
Annapolis MD 21401
Research and Development
EPA-600/S3-83-072  Sept. 1984
Project  Summary
Chesapeake  Bay Nutrient
Dynamics  Study
Jay Taft
  This study had two major objectives.
The first was to fill gaps in our under-
standing of important biological, chem-
ical,  and physical processes occurring
in Chesapeake Bay.  This  information
was  required to make the best use of
existing data and to develop future data
needs to further understand the basic
functioning of the Bay system. The
second objective was to collect a
synoptic data set for the entire Chesa-
peake Bay to be used in future modeling
efforts and to establish the  present
condition of the main Bay at one point in
  Field work for the study was conduct-
ed by  16  scientists on  13  cruises
between 1  May 1980 and 1 June 1981.
The  synoptic nutrient study was con-
ducted from 8 to 17 July  1980 in
conjunction with a circulation study
covering the entire Bay from 25 June to
29 July 1980. The process studies were
performed at various times and loca-
tions dictated by the processes them-
selves. Subsurface transport of nutrients
and  phytoplankton were  examined in
May  1980 in the upper Bay. Sediment
nutrient releases and oxygen  demand
were studied  in eight locations in
summer 1980 and spring 1981. Nitro-
gen and silica dynamics were examined
in several locations during July,  August,
and September 1980. Bacterial dynam-
ics were studied in August 1980. The
results of these studies added important
information to the knowledge about
Chesapeake Bay.
  This Project Summary was developed
by EPA's  Chesapeake Bay Program,
Annapolis, MD, 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).
  The Chesapeake Bay is a large, produc-
tive, coastal-plain estuary with 64,000
square miles of watershed and  about
4,000 miles of shoreline. Nutrients enter
the Bay directly from the shore, adjacent
urban centers, and the major  rivers
draining the watershed.
  Once in the Bay, nutrients participate
in biological and chemical cycles.  These
cycles, combined with physical circulation,
tend to trap the nutrients in the estuary.
Thus, the observed nutrient concentrations
represent the net result of many interacting
processes that comprise the biological
and chemical cycles. Estuarine scientists
and  managers recognize that nutrient
concentrations alone do  not convey
enough information. The related processes
must be  identified and placed in perspec-
  This study was planned and performed
to examine several estuarine processes
which were known to exist but were not
well quantified. It was also designed to
provide limited information about  fluxes
at the Bay mouth and a synoptic data set
for  future  modeling efforts. Time and
funds limited the processes which could
be studied to those thought to be the most
important. The  results of the field
experiments are summarized in the
following sections.

/. Synoptic Data Set
  A large body of nutrient and physical
data was collected for 8 days in July 1980
from four transects spaced along the axis
of the Bay, and from the  mouths  of the
major rivers. The data set is synoptic in
the sense that all stations were sampled
twice each day with the exception  of the
mouth transect which was sampled at 3
hour intervals  for 36 hours and  twice
daily thereafter. The purposes for obtain-

ing this data set were: 1) to establish the
current condition of the Bay with respect
to nutrients; 2) to estimate the transport
of nutrients within the Bay and across the
mouth for at least one period of time; and
3) to obtain a data set for future use in
developing a mathematical model of the
entire system.  Data collection  was
conducted within a matrix of continuously
recording current meters to enable the
transport calculations to be made.
//. Dye Tracing of Deep-Water
  A dye tracer experiment confirmed
that deep water travelling up the Bay can
reach the surface in tributaries, namely
the Chester River, and deliver nutrients,
phytoplankton, and other organisms from
down-Bay. This may be  an  important
mechanism for algal-bloom formation
and is significant in bringing larval forms
of higher species with their potential food
sources into proper nursery areas.

  Rhodamine  dye was injected into the
center of a  subsurface  lens  of the
dinoflagellate  Prorocentrum near their
annual bloom area in  the northern
Chesapeake Bay. Continuous monitoring
of the dye and density structure revealed
a net upstream movement of both the dye
and the dinoflagellates below the pycno-
  Hematoxylin stain was used to deter-
mine nuclear  division  rates for the
Prorocentrum within this subsurface-
chlorophyll maximum, showing an actively
reproducing population.

  The  strong  pycnocline repressed the
vertical advection of the dye and  limited
the upward,  phototactic migration  of
Prorocentrum.  A branching of the sub-
surface dye patch and associated dino-
flagellate concentrations were observed
at the  mouths of two major tributaries.
The dye and cells were followed upstream
in bottom waters to a  shoaling  area
where  increased vertical  advection
resulted in the breakdown of the pycno-
cline, the mixing of the dye to the surface,
and the appearance of a surface patch of
III. Sediment-Water Nutrient
  Direct measurements  of  nutrient
exchanges  between  bottom sediments
and  overlying  water confirmed the
conceptual understanding of this interac-
tion in Chesapeake Bay and, for the first
time, quantified the exchange rates.

  Oxygen and nutrient fluxes across the
sediment-water interface were measured
with  bottom chambers in four zones  of
Chesapeake Bay  (turbidity maximum,
mesohaline, near-marine, and several
tributaries) during two distinctive seasons.
  Sediment oxygen demand (SOD) ranged
from 1.04 to 4.15 g 02m~2d~1 in summer
and from 1.41  to  3.45  g Oam~2d"1  in
spring, and was quite uniform in time
and space. Calculations indicate that
SOD  is a large loss term in  oxygen
budgets (32 to 50 percent), at least in the
mid-Bay where low oxygen concentrations
are often exhibited during warm periods
of the year. Ammonium fluxes  ranged
from 102 to 669 //g at m~2rT1 in August
and from 32 to 110 HQ at rrf 2rf1  in May
and were significantly lower in the upper
Bay and lower Bay than in other areas in
August but not in May. Sediment flux
ratios of  0:N averaged 14.0 in August,
closely approximating that expected for
aerobic plankton decomposition but were
much higher (95.1) in May when NH«
fluxes were low. Evidence suggests that a
substantial fraction of the remineralized
NHj was denitrified in the sediments in
the spring but not in the summer. Nitrate
fluxes were  generally small during the
summer but were directed into sediments
and proportional to NO-j concentration in
the water column during the  spring,
again suggesting denitrifying activity.
Phosphorus fluxes were erratic in summer
(-4 to 40.2 fjg at rrT2h~1) and consistently
near zero in the spring.
  Sediment nutrient fluxes represent a
substantial  nutrient source during the
summer,  when phytoplankton demand is
high and water column nutrient reserves
are low, and a potential sink in the spring
when phytoplankton  demand is lower
and fluvial  nutrient inputs  are high.
Additional calculations  indicate that
(1) a small percentage of the paniculate
nitrogen and phosphorus delivered to the
sediment surface is sequestered in the
sediment column and (2) most of the
observed sediment nutrient   flux  is
supported by remineralization occurring
very close to the sediment surface rather
than by diagenic processes deeper in the
sediment column.
IV. Relative Nutrient
Contributions from Water
Column Recycling and
Sediment Release

  A model was developed to ascertain,
from vertical nutrient and salinity profiles,
the relative contributions of nutrients
derived from sediment release and
recycling within the water column.

  A data  set from the lower Potomac
estuary  comprised of 43 hourly vertical
profiles  of nutrients and salinity was
analyzed  and employed to develop a
model based on transport equations. The
model  is  applicable  during periods  of
relative steady-state flow such that
vertical distributions of properties do not
change during the course of measurement.
The model output consists of a dimen-
sionless parameter which indicates the
relative proportion  of nutrient in the
observed profile from water column re-
cycling  versus release from the  sedi-

  During a summer period of deep water
anoxia  in the  Potomac, the  model
indicated  that water column  recycling
was primarily responsible for the observed
profiles while release from the sediments
had a secondary  role. The technique can
be used in  other situations to ascertain the
relative importance of these two nutrient
processes  for limited time periods.

V. Sources and Sinks of Nitrite
  Direct  measurements with  heavy
isotopes quantified, for the first time in
the Chesapeake, the rates of nitrogen
transformation  between ammonium,
nitrite, and nitrate in the water column.

  The transformations of inorganic
nitrogenous nutrients that are responsible
for the frequently observed high levels of
nitrite in Chesapeake Bay and York River
were investigated using a combination of
15N tracer techniques and assays  of
chemical properties (NHi, NO2, NOa, O2,
N20, and CH4).

  During a destratification  event  in the
York River, uptake and remineralization
of NHi followed a die! cycle, but nitrifica-
tion was not as closely coupled to the light
regime. In  addition, the observed distribu-
tions of N2O, N02, and CH4 in the York^
River suggest that the source of N2O and™

NOi (both produced during nitrification)
was not in the river sediments. Rates of
nitrification inferred from NaO gas flux
calculations are consistent with measured
in situ rates.
  Experiments conducted at a series of
stations in mid-Chesapeake  Bay were
designed  to look at N-transformations
among N pools and to measure the yield
of N20-N per unit IMOa-N during nitrifica-
tion. Yield values ranged from 0.2 percent
to 0.7 percent in agreement with labora-
tory results.
  Our 1SN data indicate that oxidized N
can be formed within the water column of
the Bay when physical events cause the
mixing of NHi-rich bottom water with
more oxygenated surface layers. Surface
waters showed unexpectedly rapid rates
of NOa reduction to NHi.
  The  magnitude and duration of high
concentrations of NaO and IMOa in these
estuarine  waters during mixing  events
might be expected to increase if anthro-
pogenic  loading of nutrients causes
anoxic conditions in the Bay  to become
more widespread.

VI. Ammonium Recycling  by
  Rapid nutrient recycling within the
euphotic zone of Chesapeake Bay is a
major process that supports high rates of
production  at times  when nutrient
concentrations are low, Zooplankton,
especially those which graze  directly on
the  phytoplankton, must be closely
involved in recycling. Previous work has
shown  that the potential to recycle
nutrients increases with decreasing size
in zooplankton.  Therefore, this  study
focussed  on  the  micro-zooplankton
which  is comprised of  ciliates, rotifers,
and other organisms similar in size to the
phytoplankton themselves.

  Because of the difficulties involved in
making the necessary  measurements,
the  isotope dilution technique was
selected as  the  most suitable way to
measure  ammonium  release in the
presence of phytoplankton uptake.
  Two basic questions were asked by the
experiments: 1) At  what rates and with
what significance was ammonium being
recycled in situ in surface waters?  and 2)
What deductions could be made as to the
size of the organisms responsible for any
measurable ammonium release?

  Results indicate that the rate of
   monium  nitrogen recycling from
 Natural samples was 0.034 to 0.129/yg at
KHUN L 1h \ Further, virtually all recycling
activity was contained in the less than 35
/jm fraction, suggesting that micro-
zooplankton  had a major role in the
recycling rate.

VII. Biogenic Silica  Cycle
  An annual silicon budget was developed
for the Chesapeake Bay in this study.

  The major external source of dissolved
silicon is river input  into the Bay. A
smaller amount probably enters from the
seaward boundary. Silicon in surface
waters is utilized by organisms, primarily
diatoms. When they die or are eaten, the
siliceous components dissolve. The rate
of dissolution  of this amorphous silicon is
quite  rapid relative to that for mineral
crystalline forms (sands and clays). Some
of the particulate silicon  sinks to deeper
waters below  the pycnocline. Some
reaches  the  bottom  where  continued
dissolution enriches the  sediment inter-
stitial water producing a concentration
gradient  favoring diffusion toward the
overlying water. If the system is in a long-
term steady-state; that is, annual sinking
is about  equal to annual sediment
release, then silicon release  from the
sediments  sets the lower limit for  its
uptake in the  overlying waters.
  Measurements of benthic fluxes  were
used to focus on the role of sediment
dissolution processes in  resupplying the
water column with dissolved silicon. The
budget is in the form of a box model which
specifically excludes pools and fluxes of
mineral silicon which do not participate in
the rapid turnover affecting biological

  Calculations made from field measure-
ments indicate that sediment release of
silicon exceeds river and ocean inputs by
a factor of two to five on an annual basis.
Thus, the silicon cycle within the Bay is of
major significance relative to the external

VIII. Bacterial Biomass and
Production During Estuarine
  Bacterial abundance, biomass, and 3H-
thymidine incorporation rates were
studied during spring tidal destratification
of the York River, Virginia estuary.

  Samples were collected by pump at the
York River mouth for bacterial abundance,
by  acridine orange  direct counts  for
biomass  measurements,  and  by Niskin
bottle with 3H-thymidine for  production
measurements. Amino acid uptake was
estimated with a tritiated amino acid

  In  this system, monthly high spring
tides cause destratification of the moder-
ately stratified estuary, which oscillates
between stratified and vertically homo-
geneous conditions on a time scale of 1 to
10 days. Bacterial abundance and carbon
biomass ranged from one to eight x 109
cells L"1 and 20 to  100 /jg C L"1. Thy-
midine incorporation into cold  TCA in-
soluble fractions ranged from  1 to 10
nano-moles LT' d~1 and bacterial carbon
production rates  and specific growth
rates were seven to  75 /ug C L"1 d~1 and
0.2 to  1  L"1 d~1, respectively. Biomass
increased steadily during the destratifica-
tion process while production remained
constant. Production then increased two-
to three-fold in twelve hours  during the
period of maximum water column homo-
geneity. Increased  vertical mixing and
possible stimulation of  phytoplankton
production are hypothesized as the major
cause  of  this bacterial response to

    J. L. Taft is with The Johns Hopkins University, Chesapeake Bay Institute, Shady
      Side. MD 20764.
    James Smullen is the EPA Project Officer (see below).
    The complete report, entitled "Chesapeake Bay Nutrient Dynamics Study," (Order
      No. PB84-190 982; Cost: $25.00, subject to change) will be a valiable only from:
            National  Technical Information Service
            5285 Port Royal Road
            Springfield, VA2216J
            Telephone: 703-487-4650
    The EPA Project Officer can be contacted at:
            Chesapeake Bay Program
            U.S.  Environmental Protection Agency
            Annapolis, MD 21401
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