The Effects of Wrack Deposition and Increased Inundation Frequency on Production and Respiration in a Spartina patens/Distichlis spicata Salt Marsh





Iris C. Anderson, W. David Miller, and Scott C. Neubauer





Virginia Institute of Marine Science
College of William and Mary

Abstract

Introduction

It has been hypothesized (Brinson et al., 1995) that organic-rich "high" salt marsh may undergo a change in state to mineral "low" marsh as a result of disturbances such as wrack deposition and increased frequency of inundation. Marshes must accrete at a rate commensurate with sea-level rise; otherwise the marsh will drown and convert to open water. Vertical accretion in "high" salt marshes is thought to be primarily due to biotic processes rather than to import of sediments. Net accretion requires that gross community production exceed community respiration in the marsh. It is not clear how increased tidal flooding will affect the balance of production and respiration. The objective of our study was to determine net community production in a "high" organic-rich salt marsh dominated by Spartina patens and Distichlis spicata by measuring exchanges of carbon dioxide in the light and dark.

Methods

Studies were performed at a site developed by R. Christian and co-workers in upper Phillips Creek Marsh during 1994 to simulate the effects of increased tidal inundation and wrack deposition on organic "high" marsh. This site is normally not flooded except during high spring tides in the summer, although flooding is more frequent during winter. Water was pumped from the adjacent Phillips Creek onto the marsh during all daytime high tides. Treatments at the site included an unbordered control, a flooded site with wood border to retain pumped water, a bordered control, and a bordered, wrack-covered site. The borders allowed drainage of water at a slow rate. All treatments were replicated three times. Studies were performed bimonthly starting in August 1995 and continuing through April 1996.

Measurements at the site included net community production, community respiration, net sediment production, sediment respiration, pore water chemistry (pH, salinity, dissolved inorganic nutrients), and sediment properties (bulk density, % organic matter, % water, chlorophyll a).

Community Metabolism Measurements

Carbon dioxide exchanges were measured in the light and dark using (1) a temperature-controlled chamber (756 L) to measure total community production and respiration and (2) a sediment metabolism chamber (0.475 L) to measure microalgal production and microbial respiration. In order to scale up measurements of macrophyte productivity made over short time intervals, production (P) vs. irradiance (I) relationships were determined for each treatment and for each season. Based upon these P vs. I curves and using hourly values of irradiance from the LTER database, seasonal estimates of production were made.

In order to calculate annual sediment respiration it was necessary to determine the Q10 for each treatment. Q10 was based on the relationship between respiration and temperature using data collected over the entire 18 month study. Annual sediment respiration was then determined using the van't Hoff equation, the calculated Q10, and daily mean temperature values taken from the LTER database.

Pore Water Chemistry

Pore water lysimeters were installed within each treatment block and were sampled concurrently with carbon dioxide exchange studies. Lysimeters were purged by evacuation and then allowed to refill under vacuum for one hour. Salinity was determined using a refractometer. pH was

measured immediately following sampling. Samples for DIN were filter sterilized (0.2 µm Gelman supor filters) and kept cold until returned to the lab. Analyses were performed using an Alpkem "Flow Solution" autoanalyzer (Perstorp, 1992).

Sediment Characteristics

Sediment bulk density, % organic matter, and % water were determined for 0-2 and 2-10 cm sediment sections by standard techniques. Chlorophyll a was measured in the top 0-5 cm as described by Pinckney and Zingmark (1994).

Results

As shown in the following tables sediments at all of our plots were organic rich. pH's at the different treatment plots were all similar and slightly acidic. Salinities varied with season and between treatments. Lowest salinities were observed at the control and flooded plots suggesting that sediments at these sites were the least likely to be water-logged.


Sediment Characteristics
TreatmentBulk Density% Organic Matter
0-2 cm2 - 10 cm0 - 2 cm2 - 10 cm
C.Bordered0.125(0.022)0.147(0.036)46 (4)45 (5)
Wrack0.146 (0.029)0.151(0.035)45 (8)47 (5)
Flooded0.107(0.022)0.139(0.027)48 (3)51 (3)


Seasonal Changes in pH and Salinity
SiteAprilJuneAugust
pHSalinitypHSalinitypHSalinity
Control6.5996.53126.5615
C-Bordered6.60146.55176.4719
Wrack6.81166.64166.6918
Flooded6.58116.65216.5315

A typical P vs. I curve is shown in Figure 1 using data gathered during summer 1995 at the control site.

Figure 1


The relationship between sediment respiration and temperature for the control site was used to calculate a Q10 for the control site as shown in Figure 2. Figure 2

Total net annual community production was similar for all treatments as shown in Figure 3.

Figure 3

Flooding did not appear to affect annual community respiration in the treated plots as shown in the Figure 4.

Figure 4

Most community respiration occurred during the summer season as shown in Figure 5.

Figure 5

Annual sediment respiration was impacted by the presence of a border which reduced drainage, by flooding, and by wrack deposition as shown in Figure 6.

Figure 6


Conclusions

Based upon the preliminary results of this study we have drawn the following conclusions:

€Increased inundation frequency did not significantly affect either macrophyte or microalgal productivity.

€Wrack deposition significantly decreased rates of both sediment and community respiration

€Use of a border around experimental plots significantly perturbed rates of sediment respiration.

€Preliminary estimates of gross macrophyte production by Spartina patens and Distichlis spicata are in the range of estimates made in other studies

Literature Cited

Brinson, M. M., R. R. Christian, and L. K. Blum. 1995. Multiple states in the sea-level induced transition from terrestrial forest to estuary. Estuaries, 18 (4): 648-659.

Perstorp. 1992. Nitrate + Nitrite in Seawater. Perstorp Analytical Corporation.

Pinckney, J. L. and R. Zingmark. 1994. Comparison of high-performance liquid chromatographic, spectrophotometric, and fluorometric methods for determining chlorophyll a concentrations in estuarine sediments. Journal of Microbiological Methods. 19: 59-66.

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Submitted by ianderson@lternet.edu