The effects of salt marsh haying on benthic algal biomass.

Abstract

Salt marsh haying is a traditional activity on East Coast salt marshes and is still carried out on a large scale (over 400 hectares regularly) throughout Plum Island Sound, located in northeastern Massachusetts. The removal of approximately 90% of the aboveground biomass of the salt marsh by haying may alter many ecological processes within the salt marsh (1). One such process is the production of benthic algae. Estrada et al. (1974) found that nutrients and light are critical controls on the amount of benthic algae present (2). When a thick grass canopy shades the algae, their growth is limited not by nutrients but rather by the available light. However, when little grass canopy is present, benthic algal growth is limited by the available nutrients. Grazers are also likely to determine the amount of algal standing crop. In this project we tested the hypothesis that there should be a marked increase in benthic algal biomass after an area has been hayed because the algae is no longer limited by the available light. We took core samples at three marsh sites, each about 1 to 2 hectares in area. Two of these are regularly subjected to haying, and one is an unhayed reference area. The reference area (PUH) has not been hayed for at least 25 years. One hayed site (EPH) was last hayed two summers ago (1999). At PUH and EPH, six 1-m quadrats were placed randomly in two different vegetation zones, Spartina alterniflora (low marsh) and Spartina patens (high marsh) sites. Three quadrats in each vegetation zone at each area were cleared of aboveground vegetation by clipping, and three were left as unclipped reference quadrats. The second hayed site (HAY) was hayed in June 2001 before sampling began. At HAY, we established three 1-m quadrats within Spartina patens zones that had just been cleared of vegetation by the hayer. Six sediment cores (3-cm diameter, 1-cm depth) were taken from each quadrat at day 0, day 7, day 14, and day 30 after clipping (or haying). The six sediment cores were then pooled together into two sets of three cores. At the end of the 30-day sampling period, the aboveground plant biomass from the quadrats within the hayed and reference sites was removed to measure the regrowth of the vegetation during the experimental period. The material from the sites was then dried and weighed. The benthic chlorophyll was extracted and measured from the pooled core samples using the method of Lorenzen (3). We used HOBO HLI light intensity loggers to determine the relative amount of light reaching the sediment surface at both the treatment and reference sites. We found no difference between the benthic algal chlorophyll in the area that was hayed two summers ago and the area that has not been hayed for 25 years. Consequently, we pooled and treated the two areas as replicates in further analyses. Furthermore, we found no statistically consistent increase in algal biomass over the 30 days of the experiment in the clipped or hayed treatments. In addition, there was not a significant difference in benthic algal biomass between S. alterniflora (low marsh) and S. patens (high marsh) zones regardless of whether they were clipped or not (Fig. 1). The benthic chlorophyll concentrations in the June 2001 hayed area were significantly higher than the benthic chlorophyll in the unclipped treatments (ANOVA, F 5 3.330, P 5 0.039). However, neither the benthic algal chlorophyll present in the hayed area nor that in the reference quadrats was significantly different from that present in the clipped quadrats. The results suggest that haying on a large scale, but not small-scale removal of the plant canopy, increases the amount of benthic algae present. Based on a limited number of light measurements, there is a direct relationship between the percentage of light reaching the sediment surface and benthic chlorophyll concentrations within each quadrat (Fig. 2). In addition, it appears that there is an inverse relationship between the plant biomass of each quadrat and the benthic chlorophyll concentration (Fig. 2). 1 Governor Dummer Academy, Byfield, MA. 2 Massachusetts Audubon Society, Wenham, MA.