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Biol. Bull. 205: 248-249. (October 2003)
© 2003 Marine Biological Laboratory

Importance of Metabolism in the Development of Salt Marsh Ponds

M. E. Johnston1,*, J. R. Cavatorta2, C. S. Hopkinson3 and V. Valentine3

1 The Pennsylvania State University, University Park, PA
2 Amherst College, Amherst, MA
3 Marine Biological Laboratory, Woods Hole, MA

* Corresponding author: mej136{at}psu.edu

Ponds are a common feature on the salt marsh surface and are typically found in depressed regions of the high-marsh platform (1). They are widely held to be valuable habitat for larval and juvenile fish as well as for avifauna. Pools of standing water develop in areas that do not receive a regular supply of sediment with flooding. This standing water often becomes hypersaline and anoxic, thus inhibiting marsh grass production and further exacerbating marsh accretion (2). In areas where the rate of marsh accretion is less than the rate of sea-level rise, the formation of interior marsh ponds is expected to increase, thus contributing to marsh degradation (3).

This study focuses on pond metabolism, measured by dissolved oxygen changes, as a possible mechanism of marsh pond expansion, as well as an indication of habitat quality. A network of ponds that formed in the past 50 years in about a 1-hectare (ha) area of intertidal salt marsh was examined along the Rowley River of the Plum Island Sound estuary in northeastern Massachusetts (4).

Pond metabolism was investigated using two techniques: free-water diurnal changes in dissolved oxygen, and oxygen consumption of sediment cores. In the free-water technique, dissolved oxygen (DO) was measured in three ponds at -h intervals for durations of 3 to 5 days during June and July of 2003. DO was measured using a pulsed, polarographic O2 electrode, which is corrected for temperature and conductivity (YSI, Inc.). Rates of change in DO attributable to gross primary production and respiration were corrected for diffusion across the air-sea interface using a gas transfer velocity proportional to wind speed (5). Net ecosystem production was calculated as the balance between gross primary production and respiration over 24-h intervals.

Sediment cores (15.5 x 50 cm) were taken in the field from the edge and the center of a pond and returned to the laboratory to measure the respective benthic respiration rates. Another set of cores (10 x 50 cm) was used to determine the average carbon content of the peat in the pond environment by drying and combusting peat samples of known depth and volume and assuming a ratio of carbon to organic matter of 0.5:1. To investigate the relative lability of organic matter from varying depths, root and rhizome material (5 g wet weight) from depths corresponding to the sediment cores was placed in BOD bottles with seawater (297 ml) and a bacterial/sediment inoculum (3 ml), and respiration was measured.

Dissolved oxygen levels in all the ponds fluctuated greatly over a 24-h period, with values ranging from 0% to 200% of saturation (Fig. 1a). Along with high temperatures and salinity, this range of dissolved oxygen emphasizes the extreme conditions of these ponds as habitat. These conditions seem paradoxical in view of the assumed value of these habitats to juvenile and larval organisms.



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  		Figure 1. Patterns of metabolism in three salt marsh ponds. (a) Diurnal patterns of dissolved oxygen in Pond 3 over a 3-day period. (b) Net ecosystem production rates in three ponds. (c) Peat respiration rates by depth. (d) Benthic respiration rates in the edge and center of a pond. Center 2* indicates that the respiration rates were measured after the removal of the detrital Enteromorpha layer. P-value refers to the left two columns of the graph only.

 
The rates of gross primary production ranged from 2.75 to 6.98 g O2 m-2 d-1 in the ponds, and the rates of respiration from 3.61 to 6.91 g O2 m-2 d-1. These rates are similar to those found in the Plum Island Sound estuary (J. Vallino, Marine Biological Laboratory, pers. comm.) and eutrophic estuaries in general (6). On average, rates of respiration exceeded gross primary production in the ponds such that net ecosystem production was negative 8 out of 12 days (Fig. 1b). A negative value for net ecosystem production indicates that more organic matter is consumed than produced internally (7). The most likely source of this organic matter is the highly organic marsh sediment. Our estimates of metabolism are likely to be conservative during this time of year to the extent that anaerobic products of sulfate reduction are stored temporarily in anoxic sediments (8).

Decomposition rates of root and rhizome material decreased with depth, indicating that peat lability decreases with depth (Fig. 1c). Respiration rates in benthic cores were higher in the deep center of the pond than along the shallow edge (Fig. 1d). On the basis of root and rhizome decomposition, however, we expected to see lower respiration rates in the benthic cores taken from the center of the pond than in those taken from the edge where the remains of recently dead macrophytes were evident. Higher respiration rates in the pond center can be attributed to the presence of Enteromorpha. This alga grows around the periphery of the ponds, and its remains tend to collect in the deeper, more central portions of the ponds. Benthic respiration decreased following removal of this highly labile, detrital Enteromorpha layer from the second center core (Fig. 1d—labeled Center 2*).

The importance of excess respiration as a mechanism contributing to pond enlargement over time can be seen by comparing our measurements of net ecosystem production with our independent estimates of the rate of pond formation. Average depth-integrated carbon content of marsh peat surrounding ponds is approximately 38,446 g m-3. Assuming a maximum pond age of 50 y and an average pond depth of 20 cm (data not shown), we estimate a peat decomposition rate of 154 g C m-2 y-1.

However, two mechanisms are responsible for the increase in pond depth: peat decomposition and accretion of organic and inorganic material in the marsh adjacent to the ponds. Because the areas of marsh surrounding the ponds are not yet inundated, we can assume that the surrounding marsh has been accreting sediment at a rate comparable to that of sea-level rise, which is 2.65 mm y-1 locally (9). Accordingly, the adjacent marsh has accreted by at least 8 cm over the past 50 years. Therefore, accretion of surrounding marsh areas accounts for 40% of the depth of the ponds.

The remaining 12 cm of depth increase can then be attributed to decomposition of the inundated peat. Decomposition of 12 cm of peat over 50 years is equivalent to 92 g C m-2 yr-1 or an oxygen consumption of 0.67 g O2 m-2 d-1 (assuming a production-to-respiration ratio of 1) (7). The average rate of net ecosystem production in the ponds is -0.38 g O2 m-2 d-1. Thus, on average, respiration can account for over half the required rate of decomposition; in fact, for the majority of the observations, it can account for nearly 100% of the required carbon loss (Fig. 1b). Although our study was conducted only during the summer season, our results do suggest that respiration is an important and actively functioning process in the development of marsh ponds.

This work was supported by NSF-REU Site Grant (OCE-0097498), the Boston University Marine Program, NSF Grant OCE 9726921, and EPA STAR Grant R828677.

Literature Cited

  1. Redfield, A. 1972. Ecol. Monogr. 42: 201–237.
  2. Friedrichs, C., and J. Perry. 2001. J. Coastal Res. 27: 7–37.
  3. Kearney, M., R. Grace, and J. Stevenson. 1988. Geogr. Rev. 78(2): 205–220.
  4. NOAA Coast & Geodetic Survey. 1953. Topographic Sheet, scale: 1:10,000, National Oceanic and Atmospheric Administration.
  5. Emerson, S. 1975. Limnol. Oceanogr. 20(5): 754–761.
  6. Day, J., C. Hall, W. Kemp, and A. Yanez-Arancibia. 1989. Estuarine Ecology. John Wiley and Sons, New York. 558 pp.
  7. Odum, H. 1956. Limnol. Oceanogr. 1: 102–117.
  8. Howarth, R., and J. Teal. 1979. Limnol. Oceanogr. 24(6): 999–1013.
  9. Zervas, C. 2001. NOAA Technical Report, NOS CO-OPS 36, National Oceanic and Atmospheric Administration, Silver Spring, MD.




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