Crab Burrows, Soil Nutrients, and Spartina alterniflora : nutrients in Brownsville, VA 1992

The effect of Crab Burrows on Soil Nutrients and Spartina alterniflora by Winli Lin This study investigated the effects of fiddler crab (Uca pugnax) burrows on soil nutrients and the marsh grass Spartina alterniflora. Tall-form Spartina alterniflora (1-2m tall) typically dominates the marsh area that is flooded daily by tides. The short-form S. alterniflora(<0.5m tall) generally occupies the higher tidal heights (Bertness 1985). These short-form S. alternifloraare charterized by reduced soil drainage (Mendelssohn and Senecs 1980; Howes et al. 1981; Mendelssohn et al. 1981) and increased soil sulfide levels (King et al. 1982). From comparing control areas devoid of burrrows to those with burrows added, an increase in above-ground Spartina alterniflora production has been observed along with an increase of soil drainage rates and redox potential levels (Bertness 1985). Others have looked at how nutrient availability (Mendelssohn 1979) and sulfide accumulation (King et al. 1982; Howarth and Giblin 1983) may be the primary limiting factors controlling the production and success of S. alterniflora. While soil water movement has been shown to influence the soil parameters, (i.e., sulfide concentration and redox potential) that directly affect cordgrass production (King et al. 1982; Koch et al. 1990), little has been studied on how biotic modifications, such as crab burrows, mediate these physical factors. The Uca pugnax, are burrowing deposit-feeders that excavate and maintain semi-permanent burrows in the marsh surface. They have been found to not only oxygenate marsh soils (Howes et al 1981) and modify sediment meiofaunal abundance, they could also provide a suitable environment for continued burrowing and, as a byproduct, increase the marsh grass production and maintain the tall-form S. alterniflora. Uca pugnax, the mud fiddler crab, is the dominant form of crab seen in Brownsville, VA. Their burrows are primarily restricted to areas of tall-form S. alterniflora, due to the intermediate substrate hardness and root mat density (Bertness and Miller 1985). Therefore, the effects of burrows will be more evident when studied in areas of shorter S. alterniflorawith minimal burrow density. The objective of this study was to compare areas of minimal burrow density to those with burrows added, in shorter S. alternifloraon a mainland marsh. I hypothesized that the areas with added burrows would show: (1) a decrease in soil sulfide concentration, (2) changes in other nutrient concentrations and (3) an increase in above- and below-ground growth of Spartina alterniflora. The marsh studied was in Brownsville, VA. The site was located near the bank of No Name Creek. The focus of the experiment is on the effect of crab burrows because there were some Sesarma reticulatum burrows found. But more than 90% were created by Uca pugnax. In the first week of June 1992, 10 split-blocks were put in. Each block contained 2 subplots, one control, the other the treatment. The position of the subplots were randomly chosen. Each 0.5m x 0.5m subplot was marked off with PVC pipes at the corners. The subplots all contained 2 sippers, one to 10cm depth, the other to 20cm depth. They also all contained 3 litter bags, each one measuring the below-ground root ingrowth at 10cm and 20cm. The 10 split-blocks were in a consecutive line, separated from each other by 1m. They were situated to be parrallel to the creek bank, in the intertidal zone. To examine experimentally the influence of crab burrows, the subplots designated as the controls were each counted for burrows present and the diameter of each measured. The average diameter of the burrows was found to be 1.5cm. For each controls' respective treeatment subplot, the number of burrows found in that control was doubled for its treatment plot. These burrows were added artifically by driving a 2 ft. long 1.5cm diameter PVC pipe into the substrate. A week was allotted to allow the sippers and burrows to equilibrate and the artifical burrows to be occupied. Bertness and Miller (1984) had found that greater than 75% of their artifical burrows were occupied after a week. To asses the effects of crab burrows on sulphide, ammonia, phosphate and salinity, the sippers were sampled weekly, at rising high tide. At the end of the 9 weeks, all the Spartina alterniflora in all the subplots was cut, dried and weighed. The litter bags were removed, picked for roots, which were dried, weighed and ash dried, to find the amount of organic carbon present. There was found to be a significant difference between the controls and treatments in sulphide concentrations (p < 0.05) and in phosphate concentrations (p < 0.25). There was a significant difference between the two different sipper depths in sulphide concentrations (p < 0.01) and in phosphate concentrations (p < 0.25). In all soil nutrients measured, a significant difference in concentrations, over the 9 week period, was found (p < 0.01). The results can be found in Table 1. The above ground standing stock biomass was not statistically significant between the controls and treatments. However, the treatments were found to be 4% heavier than the controls. Thus, there is a trend towards an increase in biomass due to the treatment. The below-ground root ingrowth was not statistically significant between the controls and treatments, but the treatments were found to be 8.6% heavier than the controls. This may be an indication of an increase in growth due to the added burrows, From these results, there is evidence that crab burrows do affect the sediment chemistry and plants present. The burrows do seem to enhance the production of Spartina alterniflora and to create a more favorable environment for continued burrowing and for maintaining taller-form S. alterniflora. The addition of burrows, resulting in the decrease of sulphide concentration in situ and the altering of other soil chemistry, facilitated the aeration of the soil and increased the potential production of S. alterniflora. In all, the crabs present in the salt marsh appear to play an important role in enhancing the productivity and structure of the marshes.