The Changing Nature of Suspended Solids in Chesapeake Bay and Its Relationship to Trends in Water Clarity
Abstract
Water clarity in the Chesapeake Bay region is most commonly assessed based on either (i) visual transparency, as measured by Secchi depth (Zsd), or (ii) incident light penetration, as determined by the diffuse attenuation coefficient (Kd). Although these parameters describe different optical properties, both of these measures are important. Zsd quantifies human perception of clarity and is closely related to aesthetic value, while Kd quantifies light available for photosynthesis. Transparency (and thus Zsd) is especially sensitive to the scattering of light by particles, which makes water look cloudy, whereas Kd is relatively more sensitive to the loss of light through absorption. To fully conclude that water clarity has increased at a given location, both of these two parameters should have improved. If light penetration and transparency have trended in opposite directions, the story is more complicated.
Previous work indicates that the most important component of Bay waters determining both Zsd and Kd is total suspended solids (TSS). However, TSS - which is made up of both inorganic and organic components - is not simply sediment from land. Except near riverine or local sources of sediment from runoff, TSS is highly influenced by resuspension and/or organic detritus. The composition of TSS is also important to its effect on water clarity. It is the total cross-sectional area of TSS that most directly blocks and scatters incident light, not its mass concentration, and (all else being equal) less dense, organic detritus has a larger cross-sectional area per unit mass than does inorganic mud. So an increase in the organic content of TSS tends to decrease water clarity. The effect of less dense organics is especially apparent for Zsd, since Zsd is very sensitive to scattering. Kd is additionally sensitive to absorption, and darker colored mud absorbs more light than less opaque organics. As a result, the total effect of organics versus inorganics is less significant for Kd. Theoretically, increases in chlorophyll-a (Chl-a) should decrease water clarity because Chl-a absorbs light. Interestingly, in the Bay, high Chl-a is more strongly associated with degraded Zsd than with degraded Kd. The higher sensitivity of Zsd to Chl-a is somewhat surprising, given that Chl-a reduces clarity through absorption, and Kd is more sensitive to absorption than is Zsd. However, an increase in Chl-a is commonly associated with an increase in algal organic matter which, in turn, leads to more organic detritus. Thus the statistical association of degraded Zsd with increased Chl-a may often be due to an indirect association with increased organic solids.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMOS41C2031F
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCESDE: 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCESDE: 4235 Estuarine processes;
- OCEANOGRAPHY: GENERALDE: 4235 Estuarine processes;
- OCEANOGRAPHY: GENERAL