CICEET Progress Report for the period 9/01/01 through 1/31/02

Project Title: Modeling the effects of changes in turbidity on light available for submerged aquatic vegetation
Principal Investigator(s): Roger I. E. Newell, Raleigh R. Hood, Evamaria W. Koch, Raymond E. Grizzle

Accomplishments
Scheduled Tasks:

1. Complete the development of our mathematical model in STELLA.
2. Test the predictions of light penetration and seagrass growth from our STELLA model at a hard clam aquaculture farm on the lower eastern shore of Chesapeake Bay where turbidities may have been reduced sufficiently by bivalve filtration to allow seagrass beds to become reestablished.
3. Publish manuscripts and refine the user interface of the model based on our preliminary demonstrations to resource managers. We will also make verbal and hands-on presentations to Chesapeake Bay resource managers, at scientific meetings, and distribute the model via the internet.

Progress on Tasks

1. Continued Model Development:
   Our model simulates seston concentration, water clarity and seagrass density as a function bivalve biomass and filtration, sedimentation and sediment resuspension (Figure 1). The details of the core model structure can be found in our previous reports. Here we report on aspects of the model which have been altered and improved in the past six months.

   Specifically, we have reformulated the way in which seagrasses influence wave height, implementing a semi-mechanistic formulation where seagrass effects are parameterized as increased bottom roughness, which increases friction and wave dissipation rate. We have also recast the model so that fetch is explicitly considered, i.e., waves are propagated shore wards over a shoaling bottom with seagrasses present. The waves are progressively damped by friction against the bottom and the damping is accelerated due to increased friction where seagrasses are present.

   We have generated our first set of simulations which were designed to address two major research questions:

   1. Is there a positive feedback effect of seagrass wave damping on seagrass growth, i.e., are seagrasses self-stabilizing with respect to waves.
      Model output generated for onshore/offshore transects of Rupia density for different wave heights under conditions with and without seagrass wave damping indicate that there is a self-stabilizing effect when the reproductive shoots of Rupia are present (Figure 2). This indicates that seagrass shoot densities are significantly increased nearshore due to the wave damping effects of the shoots themselves, which reduces sediment resuspension. But this effect is only manifest when the impinging waves are relatively small (less than 0.2 m height) because the sediment resuspension by larger waves prevents seagrass growth nearshore. Thus, we conclude that there is a positive feedback effect and that seagrasses can be self-stabilizing due to their wave damping effects. However, this positive feedback effect happens only under relatively calm conditions.
   2. What is the effect of bivalve filtration on seagrass distributions and density, and is there a significant difference between the filtration effects of clams versus oysters.
      The model was run with our laboratory measured filtration rates (data reported in earlier CICEET reports) by two different bivalves, Mercenaria mercenaria (hard clams) and Crassostrea virginica (eastern oysters),for different bivalve biomasses (0 to 100 g m^-2). There were dramatic differences between the simulations with clams vs. oysters (Figure 3), where the clams have almost no impact on seagrass growth, regardless of clam density, whereas oysters have a large impact on seagrass growth even at the lowest densities. With oysters present and actively filtering the seston that increases turbidity then Rupia can grow much further inshore whereas the effect of clam filtration is insignificant. The difference between the effects of clam vs. oyster filtration is due to the fact that the filtration rate of the oysters is more than an order of magnitude higher than that of the clams.

2. Obtain data to test the model at hard clam aquaculture farms.
   In order to validate the model output we needed field data on changes in seston concentration and light attenuation associated with bivalve feeding. In the Chesapeake Bay there are few areas at the present time with sufficiently dense natural populations of bivalves that we could expect to be able to detect changes in turbidity associated with bivalve feeding. Therefore, we performed this field work over hard clam aquaculture beds on the lower eastern shore of Chesapeake Bay in cooperation with Dr. Mike Peirson, the manager of Cherrystone Aquafarms. During calm wind conditions with no appreciable wave action during August 2001 we measured changes in total seston concentration and light attenuation at 5 sites located 50 m apart along a transect across hard clam beds in Cherrystone Creek, VA. Average water depth (+ sd) at the 5 clam sites on the transect at high tide was 0.92 m + 0.05 and at low tide was 0.52 m + 0.03 m. Over the same tidal cycles we also made the same measurements at 4 sites located 10 m apart along a transect through an adjacent extensive bed of the seagrass Zostrea marina. Average water depth (+ sd) at the 4 seagrass sites on the transect at high tide was 1.1 m + 0.18 and at low tide was 0.75 + 0.16 m. three times on the flood tide and twice over the ebb tide.

   We used a flat bottom boat to drift with the tidal current and sample the 5 sites across the clam beds 3 times during the flood tide and once with the ebb tide. For the 4 seagrass sites we sampled the sites 3 times during the flood tide and twice with the ebb tide. A current meter (Marsh McBurnie) was used to record current velocity along both transects. Average current velocities were 3.0 + 1.3 cm sec^-1 and 3.8 + 2.05 cm sec^-1 for the clam and seagrass transect, respectively

   At each site we measured the diffuse attenuation coefficient for photosynthetically available light, Kpar with a 4B Li-Cor light meter positioned just beneath the water surface (O) and at 0.5 m (Z) beneath the surface. The light readings were taken twice and used to calculate the extinction coefficient K_d = [ln (light Z /Light O )] / Z from which an average Kd was calculated. We used an automated water sampler (ISCO) to collect 1 l of water at 5 cm above the sediment surface. Within 1 h of collection duplicate 400 ml water samples were filtered onto ashed and dried pre-weighed GF/C filters that were rinsed with isotonic Ammonium formate to remove salt (Armstrong 1958, Berg and Newell 1984) and then frozen. Later the filters were dried at 80°C and weighed again and the mean of the two filters used to calculate total seston (mg l^-1).

   At each location within the clam bed the size and number of clams per m^-2 was determined. Tissues from a representative sample of 50 clams were then individually dissected from shells into pre weighed aluminum pans and dried at 80 °C for 2 d for determination of each animal's total dry tissue weight. In the seagrass beds the number and size of shoots per unit area at each sampling site was determined.

   Over the 200 m length of the transect through the hard clam bed there was only a very slight decline in turbidity (Figure 4) based on the shallow slopes of the regression equations fitted to the measured seston concentrations and light attenuation coefficients. In contrast, over a distance of only 30 m within the seagrass beds, the slopes of the regression equations for these same two parameters were much steeper (Figure 5). This indicates that there was a much greater reduction in seston concentration and increase in light penetration due to the water-baffling effects of the well-developed seagrass canopy. The results of this field study are in agreement with the model predictions that hard clams have a minimal effect on seston concentration and hence do not appreciably increase light penetration through the water column. The effects of the seagrasses themselves on dampening water currents and enhancing sedimentation can be appreciable, especially under conditions of minimal wave action prevailing when we undertook the field study.

3. Disseminate Project Results We have made a strong effort to disseminate to the management and research community our insights into how bivalve suspension feeders can reduce turbidity and thereby benefit seagrasses. We have made the following presentations of this CICEET-funded research at both regional and national meetings:

   Newell, R.I.E. Potential for N, P, and Sediment Removal by Oysters. Invited presentation at an "Exploratory meeting on nutrient/sediment removal by oysters" organized by EPA Chesapeake Bay Program. Annapolis MD. March, 2001.

   We made the following co-ordinated verbal and poster presentations at a major session on seagrasses held at the November 2001 Estuarine Research Federation Biennial Meeting:

   Newell, R.I.E., E. Koch and R.R. Hood. Modeling influence of populations of suspension-feeding bivalves and seagrasses on suspended particulate load and consequent changes in light attenuation.

   Hood, R.R. M. Wood, E. Koch and R.I.E. Newell. Modeling the interaction between wave-induced sediment resuspension, bivalve filtration and seagrass growth.

   Koch, E.W. and M. Wood. Attenuation of wave height and period as a function of the vegetative state of a seagrass. Presented in the poster session

   Newell, R.I.E. Ecological value of oysters in Chesapeake Bay. Virginia Institute of Marine Science Public Seminar Series. December 2001.

Difficulties Encountered
Although the STELLA software that we have been using for model development has worked well for us in most respects, we have recently run into a fundamental limitation of this software. Specifically, we have found that it is necessary to explicitly represent fetch (distance) in our model in order to realistically simulate shoreward wave propagation and damping over a shoaling bottom with seagrasses present. This is necessary because wave damping due to bottom effects happens progressively over distance and there is a non-linear feedback effect between the wave height and the degree of frictional energy dissipation. Thus, except in the simplest cases (i.e., no change in depth and bottom friction over distance) the damping calculation has to be made numerically on a spatial grid. Unfortunately, there is no way to make this kind of calculation directly in STELLA. In order to generate the transects presented above (Figures 2 and 3) it was necessary to manually propagate the waves in toward shore by setting up more than 100 discrete STELLA model calculations where the waves are propagated and dissipated shoreward in discrete steps.

Although we plan to move forward and publish the results presented here (and at the ERF meeting) that were generated manually with STELLA, the next phase of the work will require that we recast the model in a spatially explicit form in FORTRAN.

Anticipated Success in Meeting Project Objectives in Scheduled Project Period
We have nearly met all the objectives we set out for this 24 month project. But as generally happens we still have several small aspects of the project that require completion. Based on our work plan we anticipate that we will have completed them all by August 2002.

Tasks and activities for next reporting period

Tasks for the next reporting period

1. Recast the model in a spatially explicit form in FORTRAN and carry out a full sensitivity analysis.
2. Develop the user interface of the model. Make verbal presentations at scientific meetings and hands-on demonstrations of the model to resource managers. Make any needed improvements and refine the user interface.
3. Distribute the model via the internet.
4. Write papers describing the model and the results of the field research in scientific journals.

Work plan to accomplish tasks

1. We will initiate the transfer of the existing mathematical model from STELLA to Fortran and finish the development and implementation.
2. We have been invited to make the following presentation at meetings where both environmental managers and scientists will be in the audience.

   Newell, R.I.E., J.C. Cornwell, R.R. Hood and E. Koch. Beyond Water Clearance: Incorporating Other Aspects of Benthic Suspension-feeder Ecology into Estuarine Water Quality Models. Chesapeake Bay Program Scientific Technical and Advisory Committee Workshop: "Suspension feeders: A workshop to assess what we know, don't know, and need to know to determine their effects on water quality" Baltimore, March 2002.

   Newell, R.I.E. Role of Benthic Suspension-feeders in Maintaining Estuarine Water Quality. Maryland Department of Natural Resources Seminar series. March 2002.

   Newell, R.I.E., M. K. Wood, R. E. Grizzle, E. Koch and R.R. Hood. Modeling the Influence of Filtration by Bivalve Stocks on Turbidity and Seagrass Growth. National Shellfisheries Association annual meeting, Mystic CT, April 2002.

Concerns or difficulties
Now that we have resolved that we have to reformulate the model in Fortran we anticipate no major difficulties regarding the future progress of this project.

Expenditures
In no budget categories are expenditures exceeding estimates. We have retained sufficient funds to allow us to complete the model development and undertake some limited field work over restocked oyster beds.