The Effects of Vertical Diel Thermocline Movement on the Settling of Marine Snow in a River Influenced Marine Environment
To evaluate the role of diel heating and cooling cycles in the upper water column in marine snow settling processes, particle distribution data from 15 marine snow camera casts were analyzed to examine variations in aggregate abundance, ellipsoidal volume, and particle elongation with depth. Data were collected during two cruises aboard R/V Endeavor in the Gulf of Mexico (GoM), EN620 in 2018 and EN642 in 2019. To develop a deeper understanding of daily patterns in marine snow settling, data were collected at the beginning and end of daily heating cycles. Temperature data were obtained by camera mounted Seabird SBE19+ model CTD and were analyzed for variations in thermocline depth. To focus on daily variations, the first 30 meters of the water column were studied at two-meter depth intervals. The 2019 data were collected within a distinct plume of fresh water from the Mississippi (MS) River whereas the 2018 data were collected when MS River freshwater was distributed more widely in the northern GoM. An accumulation of aggregates was seen above the diel thermocline in evening casts but not in the morning casts, caused by the stagnation of aggregates at density discontinuities. It is likely that when the diel thermocline shallows over night these aggregates are isolated below the vertical density gradient where they have a chance to settle further through the water column. Data from the three sites during cruise EN642 showed distinctly different particle abundance and volume dependent upon each site’s location within the freshwater plume.
Marine snow, Gulf of Mexico, Mississippi River, freshwater plume, diel thermocline, diel thermocline movement
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