Letter Evidence for the southward migration of mud banks in Florida Bay Kristian H. Taylor ⁎, Samuel J. Purkis 8000 North Ocean Drive, Dania Beach, FL 33004, United States abstract article info Article history: Received 16 December 2011 Received in revised form 18 April 2012 Accepted 19 April 2012 Available online 7 May 2012 Communicated by: J.T. Wells Keywords: Florida Bay Sediment Mud banks Sediment transport Migration Bathymetry Aerial imagery Geomorphology The latticework of shallow polygonal mud banks encircling deeper ponds is a key morphological characteristic of Florida Bay. Composed of lime mud produced largely by calcareous algae and epibionts, these banks limit water exchange between the interior Bay and ocean waters from both the Gulf of Mexico and the Atlantic. They also influence salinity and benthic habitat distribution. It has been proposed that the position of mud banks may be dynamic, migrating southwards with time, but no long-term study has examined the spatial arrangement of banks within Florida Bay over sufficiently long timescales to ascertain movement. Using time-separated bathymetry surveys and aerial photography datasets spanning a period of many decades, this study establishes that indeed the bank positions are temporally dynamic. The work was conducted using geographic information systems (GIS), with all data referenced to the position of relatively stable islands. The analysis reveals a southward migration trend (headings ranging from 280° to 240°) with rates averaging 1.27 m/year. For the first time in Florida Bay, the migration and vector of movement for mud banks have been documented. Despite the southward movement, mud bank morphology remained consistent. It is speculated that strong winter winds out of the north/northeast provide the mechanism for such migration. The southward migration of fine-grained, biogenic mud banks in Florida Bay demonstrates how change- detection remote sensing can be used to audit a geological process operating at time-scales of centuries. Though the available data may be unusually rich for Florida Bay, the study shows how the dynamics of other coastal systems may be accessed using a comparable work-flow. Similarly, the results here have implications for the geologic record and reevaluating paleo-landscapes where mudrock shoals have been identified. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Located at the southern limit of the Florida mainland, Florida Bay is a shallow, roughly triangular body of water that covers an area of approximately 1550 km 2 The Bay is a low-energy environment — tidal range is limited to 17 cm (Holmquist et al., 1989). The weak water circulation occurring in the Bay interior (Lee et al., 2006) is controlled primarily by wind-driven currents (Fourqurean and Robblee, 1999). Despite the relatively calm winds throughout the year, strong winter winds (November through February) have the capacity to move large volumes of water out of the Bay through tidal passes. The passes, or breaks in between the islands of the Florida Keys, are the sole locations of direct water exchange between the Bay and the Atlantic Ocean (Lee and Smith, 2002). Mud banks of Florida Bay consist primarily of biogenic carbonate lime mud originating from local calcium-carbonate producers (Stockman et al., 1967; Nelson and Ginsburg, 1986; Frankovich and Zieman, 1994). The most important of these producers are epiphytic organisms located directly on the seagrass blades of the common Thalassia testudinum. Frankovich and Zieman (1994) found the coralline red algae Melobesia membranacea and Fosliella farinosa and the serpulid worm Spirorbis sp., all of which grow on the blades of T. testudinum, to be the main suppliers of carbonate mud within Florida Bay. The calcareous algae Penicillus is also a contributor to the mud within Florida Bay (Stockman et al., 1967). The production of calcium carbonate mud is estimated at 118 g/m 2 /year for seagrass-associated epibionts (Nelson and Ginsburg, 1986) and 3 g/ m 2 /year for algae (Stockman et al., 1967). Non-skeletal precipitation of aragonite has also contributed to the lime mud budget of Florida Bay (Smith, 1940; Cloud, 1962). The morphology of Florida Bay is characterized by a network of anastomosing mud banks. With a generally flat, plateau-like surface, the mud banks of Florida Bay are shallow enough to limit flushing and water circulation from both the Gulf of Mexico and the Atlantic Ocean into the interior of the Bay (Perkins, 1977; Pitts, 1998; Smith, 2000). While the mechanism behind mud bank formation in Florida Bay is still debated, the current theory is that the banks formed along low-lying rill valleys that experienced saltwater encroachment before the rest of the Florida shelf at the last transgression; these lower areas were then colonized by mangroves that acted as current Marine Geology 311-314 (2012) 52–56 ⁎ Corresponding author. E-mail address: tkristia@nova.edu (K.H. Taylor). 0025-3227/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2012.04.007 Contents lists available at SciVerse ScienceDirect Marine Geology journal homepage: www.elsevier.com/locate/margeo