Geomorphology and sediment dynamics of the Liyashan oyster reefs, Jiangsu Coast, China
Abstract: Oyster reefs and their spatial patterns are deemed to change the local hydrodynamic condition and exert profound impacts on the grain size, concentration and transportation of suspended sediments. Meanwhile, high suspended sediment concentration often results in excess mortality among oysters. Oyster reefs are rare and vital ecosystem in Liyashan national marine park, Jiangsu Coast, China. However, urgent conservation efforts should be made on account of the drastic reduction in reef areas. To investigate the sediment dynamics and the geomorphology, two tripod observation systems were deployed and UAV aerial surveys with elevation measurement using Real Time kinematic (RTK) were also carried out. High mud content (60%) was found in the bed sediment at the reef ridge, causing much lower drag coefficient than other recorded values of living oyster reefs, indicating the death of oysters and the degradation of reefs in Liyashan. Ridgelines of the string reefs at 45° to the current direction and high suspended sediment concentration in the water body (50–370 mg/L) that exceeds the threshold (200 mg/L), which would affect nutrient uptake efficiency and further result in gill saturation, decrease of clearance rate and associated deposition, were probably crucial causes of the death of oysters. The findings are useful for restoring natural oyster reefs and designing artificial reefs for nature-based coastal defense.
Figure 1. Maps of the study area. a. Location of the Liyanshan oyster reefs in the Jiangsu Coast, China. The yellow frame signifies the area for UAV photography while the red solid denotes the tidal gauge station. b. The target reef behind the shell bank. c. The deployment scheme of two tripod observation systems. The red solid square and triangle denote the Tripod A and Tripod B, respectively. The positive along-ridge and cross-ridge directions are also defined in c.
Figure 4. Spatial patterns of the oyster reefs in the northeastern Liyashan. The red frame is the area in Fig. 1c while the blue and yellow frames signify the areas of dense aggregation of fringe reefs and patch reefs (see in Fig. 8), respectively; the black patches denote waters where lacks characteristic points for imagery reconstruction.
Figure 6. Velocities near the bed at both the reef crest (TA) and the reef front (TB) (Va, and Vc represent along-ridge and cross-ridge components respectively; straight lines are results of regression analysis and slopes of the black and grey lines denote the average directions of the flood and the ebb, respectively).
Figure 8. Spatial distribution of oyster reefs in the southwestern surveyed area (the mapped area corresponds to the yellow frame in Fig. 4 and yellow frames here denote positions of some patch reefs.
Table 1. Instrumental information and sampling scheme
Tripod Instrument Distance above seabed/m Sampling scheme Frequency/Hz Duration/s Interval/min TA ADV N5492 0.15 16 128 15 OBS 546 0.15 1 10 3 RBR 79692 0.60 1 10 3 RBR 200415 0.30 1 10 3 TB ADV N5371 0.15 16 128 15 ADV N5491 0.30 16 128 15 OBS 547 0.15 1 10 3 RBR 79836 0.60 1 10 3 RBR 200416 0.30 1 10 3 Note: Distance above seabed refers to distance from observation point of instruments to the substrate. TA: Tripod A; TB: Tripod B.
Table 2. Drag coefficients of oyster reefs
Location Drag coefficient (Cd) Reference Living reef Degraded reef Mosquito Lagoon, Florida, USA 0.031 0.016 Kitsikoudis et al. (2020) Winyah Bay, South Carolina, USA 0.032 − Styles (2015) Chesapeake Bay, Virginia, USA 0.019 0.009 Whitman and Reidenbach (2012) 0.021 − Reidenbach et al. (2013) Wreck Shoal, James River, Virginia, USA 0.010 3 0.003 7 DeAlteris (1989) Liyashan Oyster Reef, Haimen, China − 0.004 3 current research Note: − represents no data.
Table 3. Ranges of SSC in oyster reef habitats
Location Year SSC/(mg·L−1) Reference Chesapeake Bay, Virginia, USA 2012 39–65 Whitman and Reidenbach (2012) 2013 48 (mean) Reidenbach et al. (2013) Great Bay Estuary, New Jersey, USA 2017 25–32 Zappas and Moskalski (2018) Apalachicola Bay, Florida, USA 2010 14.5–79.1 Huang et al. (2016) Liyashan Oyster Reef, Haimen, China 2011 8–38 Quan et al. (2012) 2013–2014 19–136 Quan et al. (2016) 2019 50–370 current research
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