As part of a broad examination of the impact of private recreational docks on salt marsh habitat and productivity, we investigated the impacts of some structures associated with fixed docks (i.e., floating docks that rest on the bottom at low tide). These bottom-impacting structures are commonly found in the southeastern US, particularly in coastal Georgia and South Carolina, because of the high tidal range in the region (3 m at spring tide), the strong demand for water access and generous allowances for structure in the marsh to accommodate this desire. This examination is necessary because much of the understanding of impacts attributed to floating docks is based on research carried out in the Northeastern US where estuarine conditions differ significantly from those in our region.
The data from this preliminary study document that there are quantifiable effects on the benthic environment because of the presence of floating docks, which result in statistically significant changes in either benthic algal production (as measured by chl a ), grain size, organic carbon or benthic macrofaunal distributions. One impact, the decrease in benthic algal production, is a simple, direct outcome of the structure's presence over the substrate. Other secondary impacts are a more complicated result of the interaction between the dock's structural elements and other physical forces in the environment (i.e., tidal flow). These secondary impacts do not appear to be statistically associated with the whole floating structure given our present dataset. The docks examined consisted of smaller, individual floats supporting a wooden platform. The impacts are observed at a statistically significant level in the open spaces between the floats and not in the areas directly under the small floats that support the floating platform. Flow acceleration, and the associated additional erosion and removal of finer, organic-rich materials between the smaller floats is a reasonable explanation for this observation. Thus, the mechanism responsible for most impacts appears to be different than those suggested from work in the Northeastern US (i.e., wind-induced dock oscillation causing sediment resuspension or current removal of the organic-rich, fine-grained sediments stuck to the bottom of the dock).
The most obvious and direct effect of floating docks on benthic productivity is the loss of benthic diatom production (as measured by chl a) associated with dock shading of the substrate. Based on our data, total chl a content is decreased 57-73% under the floating dock. Because this impact results from shading of the substrate at all times under the dock, this impact would be felt under the complete footprint of the dock no matter the design of the dock (i.e., whether single or multiple floats support the dock).
Substrate sediment character and chemistry exhibit significant differences between the under-dock and control samples, with coarser sediments and lower organic carbon/nitrogen values observed under the docks. Where strong tidal flow is common, a flow-parallel, initial downstream fining and subsequent coarsening in grain size was observed, suggesting that bottom-impacting structures have greater effects on the benthic environment in regions where tidal velocities are more pronounced. Porosity profiles from dock and control cores suggests that the weight of the floats affects the density of at least the upper 1-2 centimeters of the seabed.
Biological data for macrofauna show that there are greater numbers of organisms and higher biomass by an order or magnitude in control sites when compared to float sites. The macrofaunal community was dominated by polychaete worms. Details of the macrofaunal distribution suggests that a predator-prey relationship may be structuring a portion of the polychaete community in our study.
In contrast, when comparing the effects of floats resting on the bottom at low tide on meiofauna, there are no obvious trends and it remains unclear if meiofauna are affected by this disturbance. In our study, the control sites had higher meiofaunal abundance and the under-dock samples lower abundance, although the differences were not statistically significant. Patchy distributions, rapid recruitment and reproductive response to disturbance of the substrate by currents and dock structures are all reasonable explanations for this lack of correlation. Meiofaunal communities in this study were dominated by nematodes, which made up 92% of the total organisms collected. Calculated meiofaunal biomass (0.16 g/m2 ) is similar in magnitude to the macrofaunal biomass found in control areas in this study (0.18-0.23 g/m2 ), highlighting the importance of meiofaunal as well as macrofaunal food resources in estuarine sediments. The distribution of these meiofaunal resources are apparently not significantly affected by dock groundings, in contrast to macrofaunal resources.
As with any biological system, the estuarine setting is complicated and requires large sample numbers, multiple replicates and repeated sampling to tease out the underlying relationships within the natural variability. Because the results presented here are based on a small number of sites, with similar types of dock construction, there need to be additional studies which examine a broader suite of samples from a greater diversity of study sites, with a more comprehensive biological sampling strategy and with greater replication among and between sites.
The three docks examined were all constructed using smaller floats under a larger wooden platform. It may be that the dominant zone of influence of a floating dock is at the interface between the float and the surrounding water (as suggested by the significant differences observed between control and interfloat sites in this study). Georgia's swift currents associated with a high tidal range may intensify the impact of floats when near the bottom. Comparative studies that examine the differences in the impacts associated with floating docks constructed on one large float and those constructed of numerous small floats could be profitably carried out to resolve what, if any, particular type of float minimizes the impact to the environment.
Combining results from a recent study of dock shading impacts on Spartina vegetation (Alexander and Robinson 2006) with results from this study characterizes the negative impact of dock and float shading on the saltmarsh ecosystem. Walkways shade the marsh, reducing biomass and carbon input by 21-37%. Adding a floating dock that rests on the bottom at low tide to the end of that walkway increases the impact of the structure by reducing the benthic algal production under the float by 57-73%. In addition, the terminal platform of the dock will shade the intertidal, non-vegetated mud-flat where benthic algae photosynthesize at lower stages of the tide, although the magnitude of the decrease from such high-standing structures was not quantified in this study and is expected to be less significant. When assessing impact to the ecosystem from docks and associated structures, both the decrease in carbon from the walkway and floating structures should be taken into account cumulatively. |
