projects

Dong Yoon Lee (이 동윤)

Estuarine Turbidity Maximum (ETM)

(Dynamic stability and particle transformations: tracing pathways of production
in Estuarine Turbidity Maxima (NSF-OCE-0453905), P.I.s: E. D. Houde, B. C. Crump, E. W. North, M. R. Roman, L. P. Sanford, Senior Investigators: S.-Y. Chao, R. R. Hood, D. Kimmel)

Fig. 1 Conceptual diagram of estuarine turbidity maximum regions (gray area). Arrows indicate the direction of water flow; freshwater above and seawater below the pycnocline which is shown with a line.

The ETM can be found worldwide anywhere seawater meets freshwater (Fig. 1). Less dense freshwater flows over the top of seawater and it creates a sharp gradient of salinity called a pycnocline. At a front head of seawater we typically measure the highest concentration of suspended sediment particles because of friction between the pycnocline and sediments. Due to the dynamic stability of ETM circulation patterns, most suspended particles are trapped in this region, and that is why we call the region the estuarine turbidity maximum.

The ETM supports the highest secondary production, including zooplankton, larval and juvenile fish, in estuarine environments because it traps organic matter and hydrodynamically concentrates prey items for secondary producers. For these reasons, striped bass (aka. rock fish) migrate from the Altantic Ocean to the Chesapeake Bay ETM region every year in April and May to breed and spawn eggs. There are at least three reasons why they spawn eggs in the ETM:
(1) Optimal density for their large bouyant eggs
(2) Reduced predation stress
(3) Abundant food items

Our main objective of this study is to identify pathways of energy flow from the delivery of organic matter to the production of larval and juvenile fish. Our overarching hypothesis (Fig. 2) is dynamiy ETM circulation increases the quantity and quality of prey items (e.g., microaggregates, zooplankton) for secondary production.

Fig. 2 Overarching hypotheses covering the delivery of organic matter and microbial organisms up to the highest trophic level organisms.

In the ETM project, I was in charge of measuring community metabolism, including primary production and community respiration, and developing ecosystem food web models. Primary production and respiration are the most basic form of biological processes conducted by all living things and we can estimate how much energy is produced and consumed. For example, net ecosystem metabolism, calculated by subtracting community respiration from gross primary production, is often used to estimate whether the system is dependent on organic matter produced in the same ecosystem (autochthonous) or imported from external sources (allochthonous).

Fig. 3 Energy flow diagram illustrating the estuarine food web in the Chesapeake Bay estuarine turbidity maximum (ETM) region. FW: fresh water; SW: sea water; POM: particulate organic matter; DOM: dissolved organic matter; dotted line: pycnocline. Details are discussed in Lee et al. (2012).

My research indicates that heterotrophic organisms in the ETM are strongly dependent on both autochthonous and allochthonous organic matter but with strong seasonal variation (Fig. 3). It is because the strength of the ETM is highly associated with river discharge which usually peaks in early spring. Therefore, the contrubition of particle-attached and free-living bacteria was relatively higher than that of phytoplankton production in April when the ETM was the strongest. Most interestingly, however, dinoflagellates appear to be the greatest contributor for both primary production and community respiration because abudant dinoflagellates (e.g., Heterocapsa rotundatum, Prorocentrum minimum) are mixotrophs that can live (or switch) to both autotrophic and heterotrophic life styles.

Fig. 4 Nutrient-Phytoplankton-Zooplankton-Bacteria model was prepared (Stellar software) to test a hypothesis suggesting that the contribution of dinoflagellates as both primary and secondary producers is a key mechanism resulting in efficient energy transfer to higher trophic level organisms.

I developed a NPZD (i.e., nutrients, phytoplankton, zooplankton, detritus) model using STELLA software to simulate how the physiological advantage of dinoflagellates causes highest secondary production in the ETM region (Fig. 4). Model results suggest that dinoflagellates transfer bacterial and detrital organic matter efficiently to omnivorous estuarine copepods that are heavily grazed by fish larvae.

I have mainly talked about biological mechanisms resulting in high secondary production. However, it is difficult to explain such high secondary production without considering physical mechanisms concentrating all kinds of small particles in the ETM. So, I developed another model, called the Box model, to create a simulation of ETM physics (Fig. 5). I parameterized the model with actual values of various envioronmental measurements such as phytoplankton concentrations, river flow, water advection, diffusion, mixing, and most importantly sinking rates which were acquired from the Owen Tube experiment. The model results suggest that the function of ETM physics is also critical to increase the concentration of labile organic matter in the ETM regions.

Fig. 5 Schematic diagram of the Box model structure. The oligohaline region of Chesapeake Bay has longitudinally separated into three regions representing the upstream, ETM, and downstream.

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