Virginia Institute of Marine Science
14th International Conference on Cohesive Sediment Transport Processes (INTERCOH), Montevideo, Uruguay
Biogenic pelletization plays an important roll in the packaging of fine sediments to prevent their availability in contributing to the water clarity issues in coastal systems. On the order of 1000 5µm equivalent spherical diameter clay flocculi primary particles can be packaged in a single elliptical-inshape pellet, 100µm long and 30µm wide. This is a size consistent of those observed in our study area in the Clay Bank Area of the York River, a tributary of the Chesapeake Bay, Virginia, USA. If resuspended, this one pellet will have almost 25 times less surface area and thus have that much less impact on water clarity, than its constituent primary particles. Biogenic pelletization can be significant where there are large numbers of organisms that suspension feed, pump quantities of water into their feeding systems and extract suspended solid particles. It can be even more significant when the pellets are resistant to degradation and breakup under normal tidal stress. For example, one study of Callianassa major, a ghost shrimp, estimates it produces 2,480 medium sand size resilient pellets per day, which collect and are transported in the troughs of sand ripples (Pryor, 1975). In the study, Pryor estimates in an area of the Mississippi Sound, where the density of Callianassa burrows approach 500/m2 , they can remove and pelletize nearly 618 metric tons of suspended solids per square kilometer each year. “Enough to cover each square metre of the back island shoreface with 141 mm of fecal pellets each year” (Pryor, 1975). The resilient pellets in our study have been found to comprise up to 40% of the bottom sediments in the Clay Bank Region of the York River, Virginia, USA (Rodríguez-Calderón and Kuehl, 2013). From video collected by Robert Diaz’s WormCam, we believe these resilient pellets are produced by polychaetes. The burrows and fecal pellets, though smaller, are very similar to those described of the polychaete Nereis diversicolor in the Kundalika Estuary on the west coast of India (Kulkarni and Panchang, 2015). A nearly continuous series of sediment trap deployments on bethic tripods from September 2012 through March 2015 show a loosely seasonal trend (dotted line in Figure 1) in the pellet concentration of the homogenized sediment collected in each trap with inlet holes 0.4 mab. X-rays of the traps show that the sediment collected, however, is not homogenous. They show thin laminations of coarser material throughout the trap, which we hypothesize to be tidally resuspended pellets, sand and coarse debris. Two additional sediment traps were deployed on a tower in conjunction with the deployment of the benthic tripod deployed August to November 2016. The surface trap’s 1cm diameter inlet holes were within 2m of the surface, and the bottom trap’s were at a mid-water depth of ~3.2m above the bed (mab) (referred to as mid-trap). The pellets were separated using a 90µm and a 63µm sieve. The sediment collected in each of the three traps are visually similar in the microscopic photos taken (Figure 2). There was more debris in the mid and tripod traps than in the surface trap and some very large pellets were visible in the mid-depth trap. Each trap contained over 99% mud, and the percentage of mud packaged as pellets in each of the traps was 2.00, 4.05 and 4.89 for the top, mid and tripod traps, respectively. This shows that these pellets are being resuspended and transported throughout the water column. This is consistent with the findings of Haven and Morales-Alamo (1968) during a study in the James River, an adjacent tributary of the Chesapeake Bay, and suggests that sorting during transportation and deposition could result in accumulation in areas of the estuary where fine sediment would not settle out. Two 4-6 hr profile cruises, one around max flood and the other around max ebb, were conducted early in the deployment of the sediment traps. The instruments mounted on the profiler included a Particle Imaging Camera System (PICS), a LISST 100X, a YSI EXO sonde and a pump for water samples. Future work will include identifying pellets in the PICS video sequences using axis ratios and pixel intensity to differentiate them from like size particles
Massey, Grace; Fall, Kelsey; Friedrichs, Carl; and Smith, S. Jarrell, "Evidence of muddy aggregates as resilient pellets in suspension throughout the water column using traps and a Particle Image Camera System (PICS) in a tidal estuary" (2017). Presentations. 54.