Interactions of Estuarine Physics, Sediment, and Organic Matter in Determining Suspended Particle Properties, Their Spatial and Temporal Distribution, and Resulting Water Clarity

College Of William & Mary Virginia Institute Of Marine Science, Gloucester Point VA

Investigators

Abstract

Water clarity in coastal and estuarine systems impacts ecosystem dynamics, influences essential habitat for key life stages of many species, and is also valuable for aesthetics and recreation. In Chesapeake Bay, water clarity has seemed particularly unresponsive to management practices, and has continued to decline over recent decades. Paradoxically, clarity has decreased more in areas with relatively low concentrations of inorganic sediment than in locations where riverine sediment loads have increased. Here it is proposed that the apparent disconnect between water clarity and input of sediment in the Chesapeake Bay and many other estuaries is related to the interaction of common estuarine flow patterns with the presence of excess organic matter. Through improved observation, modeling, and analysis of the interaction of estuarine physics with suspended organic matter and inorganic sediment, this study aims to transform understanding of the controls on water clarity in estuaries. States in the Chesapeake Bay watershed will invest billions of dollars over the next decade to further reduce runoff of nutrients and sediment. In the future, analogous investments may be required for estuarine watersheds nationwide. By evaluating how estuarine physics, organics, and sediment interact to influence water clarity, this study will provide valuable guidance on effectively investing these funds. This project will fund two PhD students on societally-relevant dissertation projects and provide engaging projects for undergraduate researchers. In many coastal settings, the interactions that determine water clarity are sensitive to particle concentration, size, density, composition, organic content, and settling velocity, yet few studies have explicitly considered how these connect to generalized estuarine turbulence and circulation patterns. The main objective of the study is to better understand the interaction of small and large scale estuarine physics with mixtures of inorganic sediment and organic matter typical of partially-mixed systems. Novel field observations and numerical models will be developed to test these ideas and focus on the York River estuary, a logistically attractive, broadly representative, partially-mixed branch of the Chesapeake Bay. Field observations will characterize physical oceanography (circulation, stratification, and turbulence), as well as particle populations (including size distribution, concentration, density, settling velocity and organic constituents). Open-source, community-supported numerical models will be implemented to further investigate feedbacks between estuarine hydrodynamics, organic content, and sediment dynamics, incorporating multiple particle sizes and densities, and new formulations for composition-dependent flocculation, as well as state-of-the-art formulations for cohesive bed behavior. This work will test the following novel hypotheses: (H1): Addition of organic material further enhances the horizontal and vertical particle sorting that characteristic to the physics of partially-mixed estuaries. (H2): Organic matter added to low concentrations of inorganic solids results in relatively small, low density flocs having slow settling velocities (Type 1 flocs). Conversely, adding organic matter to high concentrations of inorganic flocs favors larger, higher density flocs, with increased settling velocity (Type 2 flocs). (H3): Organic rich flocs (Type 1) are especially effective at attenuating sunlight because they are suspended high in the water column and have a large cross-sectional area per unit mass. The end result of interactions with estuarine physics is that the addition of organic matter to the estuary degrades water clarity in the lower estuary and improves water clarity in the upper estuary, closer to the estuarine turbidity maximum.