Commentary of Strategies for Enhancing Marine (and Human) Habitat at Brooklyn Bridge Park

Benthic invertebrates are generally not affected one way or the other by light, since for most of the ocean sunlight does not penetrate through the overlying water to the seafloor. If the water is shallow and clear enough to allow sunlight to reach the seafloor, however, the effects of shading may become important. There are 2 main issues to consider for whether shading associated with the proposed floating walkways at Brooklyn Bridge Park will affect benthic invertebrates.

The first issue is direct versus indirect effects. Light is important to visual predators, such as many species of fish, but most kinds of benthic invertebrates (worms, mollusks, echinoderms) have limited mobility and do not use vision for finding food. For them, shading or even the complete absence of light will have no direct effect on getting food. Other kinds of benthic invertebrates, such as crabs or shrimp, are mobile scavengers or predators, but they use mainly chemical or mechanical cues rather than vision for locating food. Shading could even be advantageous for them, if it decreases the effectiveness of their own visual predators. Indirect effects, however, are another matter. In water shallow and clear enough to allow sunlight to reach the seafloor, growth of plants, such as benthic microalgae or submerged aquatic vegetation, can be a substantial source of food for benthic invertebrates. Reductions in light intensity lead to direct reductions in plant growth (MacIntyre et al. 1996) and the potential for eventual reductions in food supply to benthic invertebrates. Stocks and Grassle (2001), for example, found that shading salt marsh ponds reduced the abundance of benthic invertebrates by 62% via a reduction in the amount of benthic plant food.

The second issue is how much shading will result from the floating walk-ways, and will it have an effect (actually, before even considering that, how much sunlight reaches the bottom already)? The average mean water depth where the walkways would be built is 12.2 m. Turbidity, as well as water depth, affects light transmission. If very little light is already reaching the bottom because the water is highly turbid, the issue of shading due to the walkways is a moot point. A study by Struck and colleagues (2004) on the effect of bridge shading on North Carolina marshes provides some guidance. The bridges they studied were of different widths and heights above the water, leading to different amounts of shading. They found that reductions in average light intensity by 83–89% were correlated with significant reductions in the biomass of salt marsh plants, the organic matter content of sediment, the total abundance of benthic invertebrates, and the number of species of invertebrates. These shading effects occurred when the height-to-width ratio was less than 0.68. Although the difference in setting makes it tenuous to apply Struck and colleagues’ results to the floating walkways proposed at Brooklyn Bridge Park, 2 points are worth considering. One would be to not build floating walkways but raise them so that the shading effect is lessened. Based on the proposed width of 6.2 m and using Struck and colleagues’ cut-off, a mean height of 4.2 m above water might be sufficient. Aesthetic values and costs would need to be considered. On the other hand, the design as proposed might not produce enough ‘average’ shading to be of concern. Some data on the amount of light attenuation that the design would produce, at the actual site, would be of value.