MEERC FACILITIES MEERC FACILITIESThe Center is organized around program elements that use a combination of physical (experimental ecosystems) and mathematical models to explore scale-dependent ecosystem responses to external forcings:
Ecosystems of the land-sea interface (wetlands to open estuarine waters) have been the subjects of the Center's research. Experimental ecosystems, small scale representations of natural ecosystems or "cosms", provide an experimental tool for comparative studies of ecosystem performance. Interactive modeling and experimental manipulations of cosms are used to examine how the structure and function of ecosystems change over a range of scales in response to external perturbations. Statistical, simulation and network models have been developed and used to formulate hypotheses, predict ecosystem response across scales of variability, and to develop methods for assessing the health of ecosystem in nature.
Mesocosm Facilities
The use of experimental ecosystems as a tool for the study of scale-dependent ecosystem behavior was essential to achieving the goals of MEERC, and a major investment was made to design systems that provide the means to control, mimic, and monitor levels and patterns of turbulence, groundwater flows, and tidal fluctuations that organisms experience in nature. Given the short generation times of planktonic organisms and the desire to run experiments at any time of year, we developed indoor pelagic/benthic (P/B) mesocosms that would allow control of incident radiation and temperature. Our submersed aquatic vegetation (SAV) mesocosms were also located indoors. Given the annual nature of the growth cycles of marshes, we designed these systems for outdoor use.
We designed a flexible system of P/B mesocosms that permits changes in tank configuration, mixing regime, lighting, and temperature. A state-of-the-art system supplies unfiltered and filtered (to 0.5 mm) water from the Choptank River estuary (mesohaline), full strength sea water, well water (fresh), and mixtures of the above as desired. The current tank configuration includes fiberglass tanks ranging in volume from 0.1 to 10 m3 in five different shapes. Up to 33 tanks have been operated simultaneously. The mesocosms are exposed to cool white fluorescent light (up to 200 mE M2 s-1) that is uniformly distributed over the surface of each tank. Mixing systems consist of rotating, reversing paddles, with speed, direction, and duration controlled by computer to simulate mixing as modulated by tides in nature. In addition to P/B experiments, the tanks have been used for studies of SAV growth under different mixing regimes and as pelagic systems for benthic boundary layer studies (linked pelagic and benthic flume systems).
During P/B experiments, continuous measurements of light, temperature, and dissolved oxygen can be made with in-situ sensors linked to a computer data-logging system. A cart-mounted water quality system uses a water pump to sample routine water quality data (in vivo chlorophyll a, salinity, temperature and dissolved oxygen). Filtration and sample preparation are made in an adjacent water quality laboratory and routine biogeochemical analyses are performed by HPL analytical services.
Substantial effort has been invested in the automation of measurements and data acquisition as a means of increasing the temporal resolution of measurements, streamlining the collection and processing of data, and standardization of analytical methods. Targeted variables are humidity and air temperature, incident and downwelling radiation, water temperature, pH, salinity, dissolved oxygen, chlorophyll, turbidity, and macrozooplankton.
Separate marsh and SAV mesocosm facilities were established to explore the effects of groundwater flows on marsh growth and nutrient retention and the effects of variable nutrient supplies and trophic complexity on the growth of SAV and nutrient retention.
A set of 9 sandy fringe marsh mesocosms and a set of 12 larger interior marsh mesocosms were established, each with a high marsh (Spartina patens) and a low marsh (S. alterniflora) and with controlled groundwater and tidal flows and associated nutrient inputs. The mesocosms were 3 m long (sandy fringe) and 6 m long (inland marsh), and were constructed of 2.5 mm thick fiberglass walls. Tidal control involved a digitally controlled system, whereby tidal height data were input to a computer that controls inflows and outflows. Tidal amplitude was monitored using specially constructed pressure sensors.
Groundwater inflows were valve-controlled. Two constant head tanks (3m) supplied high nitrate and R/O water. Initially groundwater was obtained from shallow wells, but as the volumes increased in our systems, it became more economical to produce nitrate-rich water from the R/O supply. The groundwater was chilled to 55EC in the holding tanks to keep the root zone lower in temperature during hot days than the ambient air temperature. This produced less root respiration and higher gross primary production of macrophytes. In contrast to the groundwater, the tidal water was not synthetic but was derived from the Choptank River estuary. It was stored in 3 m3 tanks where algal growth was allowed to take place to make up for pipeline losses.
The SAV mesocosm facility consisted of 16 rectangular tanks (1 m3) tanks located in a temperature- and light-controlled facility. Each tank received unfiltered or filtered water directly from the Choptank River and from wells, as well as water that had been scrubbed of nutrients from adjacent SAV ponds. Tanks had separate circulation capabilities, and water could be moved in both directions to simulate the tides. Adjacent to the SAV mesocosm facility were 6 ponds that contained 4 species of macrophytes. These provided plants used in mesocosm studies and served as nutrient scrubbers for brackish river water used in the SAV mesocosms.
We have designed and tested an Ecotoxicology "SToRM" mesocosm that has realistic
benthic and water column turbulence levels in a single system. The facility
allows realistic tidal and/or storm sediment resuspension and overcomes the problems of current
mesocosm designs that are depositional environments with unrealistically low
bottom shear and no sediment resuspension.
We are using the new system to test hypotheses relating to the effects of
organisms and water flow on nutrient and contaminant cycling in ecosystems. All
experiments to date have been performed with contaminated muddy sediments subjected to tidal
sediment resuspension versus non-resuspension. The effects of resuspension and
non-resuspension on the ecosystem and the nutrient and contaminant dynamics are
followed under controlled comparative ecosystem experiments with triplicate
treatments.
The main objectives of the mesocosm studies in 2001 and 2002 have been to:
1. Test our new experimental ecosystem design with tidal resuspension and
realistic internal mixing energies by
comparing material and contaminant cycling processes in these systems to
standard isolated tanks that have artificially low flow at the sediments.
2. Determine the effect of benthos and tidal resuspension on nutrient and
contaminant cycling in ecosystems, on ecosystem processes, and on the bioavailability and
bioaccumulation of contaminants into benthic organisms. In addition, we are
hoping to determine how
increased ecological complexity affects the results.
We have performed three controlled comparative ecosystem experiments with the
SToRM mesocosm facility (which have worked very
well) and are analyzing the data.
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