Coastal sands as gigantic filter systems
Markus Huettel, Ph. D.
The shallow shelf environment at the interface between land and the deep ocean stands out because of its extreme productivity. Plankton blooms provide nutrition for crustaceans and fish, advancing the shelf to the most important region for global fisheries, but plankton blooms also cause toxic red tides and massive fish kills.

High organic matter input from land is mineralized in the shelf to nutrients which add to the already large freight of nutrients transported into coastal waters by rivers, land runoff, and precipitation. A key compartment in which microbial mineralization processes occur is the marine sediments. In the shallow water, a large fraction of the organic particles settle to the sea floor where they are embedded in the sediments. The most common sediment type in the coastal zone is sand, covering approximately 70% of the global continental shelf.

The research group led by Dr. Markus Huettel in the Florida State University Department of Oceanography investigates the role of coastal sand beds in carbon and nutrient cycling. White sand beaches make Florida a prime tourist destination, and we can observe the movement of the sand grains and resultant ripple formation when we swim and snorkel in shallow water. This ripple formation and the flow of water currents over these ripples convert the coastal sand beds into gigantic filter systems.

When bottom currents move over a rippled sand bed, the deflection of the flow by the ripple topography (which develops with ripple crests perpendicular to the main flow direction) generates small pressure gradients at each ripple, similar to air flow over the wing of an airplane. This gradient--with lower pressure over the ripple crest and higher pressure in the ripple troughs--drives water along curved paths through the upper layers of the sea bed.

Up to 1000 liters of water are pumped through each square meter of sandy sea floor each day. Substances dissolved in the water, like oxygen, nitrate and dissolved organic matter are transported with this water flow into the sediment, and even particles that are small enough to fit through the pores are carried into the sand bed. This particulate material is removed from the pore water flow through the same filtration process that used in a swimming pool sand filter, and the trapped particles are then degraded by a large spectrum of organisms that live on or between the sand grains.

Likewise, the dissolved organic matter is used by the microbial community in the sand, and this sedimentary degradation is much faster than in the overlying water column because the number of bacteria in one cubic centimeter of sand exceeds the number of bacteria in the water by several orders of magnitude. Oxygen flushed through the upper sediment layers with the pore water flow accelerates the mineralization process. As the same volume of water that is pushed into the sand has to be released again, a steady flow of pore water loaded with the products of the sedimentary decomposition processes (e.g. inorganic nutrients, carbon dioxide) emerges the sand beds. Thereby, the nutrients that are released through the mineralization of the sedimentary organic matter become available again for the algal production at the sediment surface and in the water column.

Dr. Huettel's research is featured in FSU's award-winning magazine, Research In Review, in the article "The Living Sands of St. George"

• Huettel, M, P Cook, F Janssen, G Lavik, JJ Middelburg. 2007. Transport and degradation of a dinoflagellate bloom in permeable sublittoral sediment. Marine Ecology-Progress Series 340: 139-153.
• Cook, PLM, F Wenzhofer, RN Glud, F Janssen, M Huettel.  2007. Benthic solute exchange and carbon mineralization in two shallow subtidal sandy sediments: Effect of advective pore-water exchange.  Limnology and Oceanography 52: 1943-1963.
• Meysman, FJR, OS Galaktionov, PLM Cook, F Janssen, M.  Huettel, JJ Middelburg. 2007. Quantifying biologically and physically induced flow and tracer dynamics in permeable sediments.  Biogeosciences 4: 627-646.
• Reimers, CE and others 2004. In situ measurements of advective solute transport in permeable shelf sands. Continental Shelf Research 24: 183-201.
• Huettel, M, A Rusch. 2000. Transport and degradation of phytoplankton in permeable sediment. Limnology & Oceanography 45:  534-549.
• Huettel, M, IT Webster. 2000. Porewater flow in permeable sediment, p. 144-179. In BP Boudreau and BBJørgensen [eds.], The Benthic Boundary Layer: Transport Processes and Biogeochemistry.  Oxford University Press.