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ENVIRONMENTAL QUALITY AND PRESERVATION— Reefs, , and Carbonate Sands: Guides to -Ecosystem Health and Environments

A healthy reef ecosystem builds a protective offshore barrier to catastrophic wave action and storm surges generated by tropical storms and hurricanes. Introduction

In recent years, the health of the entire reef ecosystem that lines the outer shelf off the Keys has declined markedly. In particu- lar, loss of those coral species that are the building blocks of reef framework has significant negative implications for economic vitality of the region. What are the reasons for Aerial photograph of spurs and grooves at Sand Key Reef, located southwest of , Florida. Spurs and this decline? Is it due to natural grooves are oriented perpendicular to incoming waves. Spurs are approximately 6 m (20 ft) wide. change, or are human activities (rec- reational diving, ship groundings, United States and other parts of the morphology, and their varied farmland runoff, nutrient influx, world. natural response to rising level. air-borne contaminants, groundwa- In cooperation with the National These studies also assess movement ter pollutants) a contributing factor Oceanic and Atmospheric Adminis- and accumulation of sands, relative and if so, to what extent? At risk tration’s (NOAA) National Marine to direction of prevailing energy, and of loss are biologic resources of the Sanctuary Program, the U.S. Geolog- origin of the component sand grains. reefs, including habitats for endan- ical Survey (USGS) continues long- Geophysical data collected with high- gered species in shoreline man- term investigations of factors that may resolution -wave instruments groves, productive marine and wet- affect Florida’s reefs. One of the first that provide pictures of the land nurseries, and economic fisher- steps in distinguishing between natu- and bedrock are used to interpret ies. ral change and the effects of human sediment thickness. Reef thickness is A healthy reef ecosystem builds activities, however, is to determine determined by collecting a protective offshore barrier to cat- how coral reefs have responded to cores by drilling. Drill cores astrophic wave action and storm past environmental change, before the through reefs are used to identify the surges generated by tropical storms advent of man. By so doing, accurate coral species that built them and to and hurricanes. In turn, a healthy scientific information becomes avail- determine how reefs reacted to rising reef protects the homes, marinas, able for Marine Sanctuary manage- sea level. These data are supplemented and infrastructure on the Florida ment to understand natural change and by using isotope-dating techniques to Keys that have been designed to thus to assess and regulate potential derive the carbon-14 (C14) age of the capture a lucrative tourism industry. human impact better. The USGS stud- corals and mangrove in the cores. A healthy reef ecosystem also pro- ies described here evaluate the distri- Mangrove peat forms in very shallow tects inland agricultural and live- bution (location) and historic vitality water and at the shoreline but is found stock areas of South Florida whose (thickness) of Holocene reefs in South today buried beneath offshore reefs. produce and meat feed much of the Florida, relative to type of underlying

U.S. Department of the Interior USGS Open-File Report 00–164 Background Information Pleistocene reefs may seem at first Holocene topography mimic those of Geologic time is classified into glance to be a reflection of com- the Pleistocene, i.e., Holocene reefs various categories, one of which is an paratively less time for development. began growth on top of Pleistocene epoch. The Pleistocene Epoch spanned However, Holocene growth was con- reefs, and uncemented Holocene sed- time from ~1.77 million (1.77 Ma) strained primarily by a slowing of the iments overlie cemented Pleistocene to 10,000 years ago (10 ka) and was rate of sea-level rise that limited space . Some reef cores contain a period of alternating glacial and for upward expansion and, in Florida, mangrove peat at the bottom that interglacial stages with corresponding by flooding of the inner shelf at ~2 formed when the bedrock shelf sur- rapidly fluctuating high-amplitude sea ka that created Florida and Biscayne face was exposed in the early levels. The last glacial interval, or Bays. Rising sea level altered water Holocene. Presence of peat offshore Ice Age, occurred at the end of the depth, which affected wave energy indicates an earlier shoreline position Pleistocene when sea level is believed and circulation and changed water and a period of lower sea level. C14 to have been more than 130 meters chemistry and temperature. Sea level dating of the peat tells us when the below present (1 m = 3.281 feet). at 2 ka was ~0.5 m lower than old shoreline existed, and location of The Holocene Epoch followed, begin- today, and tidal influx of deleterious the peat ‘x’ meters below present sea ning about 10 ka, and extends to the murky, nutrient-rich water and level tells us the depth of the previous present. The first ~3,000 years of the cold of Mexico water to the clear shoreline relative to present sea level. Holocene were marked by a period warm waters surrounding the reefs of rapidly melting Ice Age glaciers had begun. Reef Records and warming oceans. The rapidity at The Holocene coral species recov- which these events occurred caused an Environmental Signatures in Reefs, ered in cores are framework builders. initial rapid rise in the most recent Corals, and Carbonate Sands Many single cores from individual sea-level fluctuation. Sea-level curves reefs contain species of head corals derived from isotope dating of corals By examining the evolutionary at the bottom and branching corals at and mangrove peat relative to their of the reef-tract eco- the top, indicating a change in envi- locations below present sea level indi- system, we can understand what con- ronmental conditions at that location cate that the rapid rate of Holocene trols where and why reefs grow. through time. An analysis of a number sea-level rise began to slow about 7 Because coral larvae generally settle of cores from transects across a reef ka. on sites where the bottom is hard shows the same coral zones through In South Florida, the bedrock and free of sediment, reefs began time and space, i.e., seaward head- beneath the Florida Keys and reef growing on elevated bedrock. Sands, coral zones shifted landward to tract is Pleistocene. Some Pleistocene which prevent reef establishment, branching-coral zones within the same reefs in Florida are as much as filled depressions. To grow, corals reef. The shift occurred as the reef 30 m in height, 0.5 kilometer (1 require narrow conditions: relatively migrated or grew landward (called km = 0.6214 mile) in breadth, and shallow depths and normal salinity, backstepping) over carbonate sands extend the length of the reef tract, clear nutrient-poor water, and warm with rising sea level. In both exam- indicating optimal environmental con- temperatures. The primary control of ples, the massive head corals grew ditions for coral growth during that these conditions, in other words, reef first, indicating that quiet-water con- time. Pleistocene reefs are cumulative growth, is sea level. The primary con- ditions (> 5 m) prevailed first. in that they alternately grew, died trol of reef distribution is the availabil- Branching forms that prefer the surf from exposure during lowstands of ity of a hard, elevated, sediment-free zone (< 5 m) then replaced the head sea level, and were re-colonized by surface, in other words, the landscape corals. C14 ages on both types of corals during the next rise in sea level. or topography of the bedrock under corals give us the times in the area By contrast, Holocene reefs flourished the reefs. In Florida, the bedrock sur- where the cored reefs are located continuously during the latest rise in face was dry land until about 7 to 6 ka when sea conditions were quiet and sea level and attained a maximum when the sea began to flood what is then became high energy. The coral of 16.8 m in thickness within 5,000 now the reef tract. zones developed in response to flood- years, indicating that conditions for Much can be learned about ing of Pleistocene-reef that coral growth then were also excellent. Holocene reefs, corals, sediments, and once lined the edge of the margin C14 ages show that the reefs lived environments by core drilling through during the early period of lower from about 7 to 2 ka. The fact the reefs. Coral-rock cores show that, Holocene sea level. The reef islands that Holocene reefs are thinner than in general, the components of had protected the head corals from

2 Healthy ecosystem components—head and branch- ing reef-building corals in clear Florida water in 1971. Tiny coral polyps, soft-bodied animals living in colonies like bees, build coral reefs. Each polyp constructs an outer skeleton of hard limestone that adds a new layer to the reef. Diameter of head coral is approximately 0.7 m.

m = meters

high-energy waves. Their submer- Today, outer-shelf reefs off the upper Coral Records gence removed that protection and Keys are protected by the linear Individual corals also contain allowed the to move land- of Key Largo not from high wave environmental signatures—within ward, creating the high-energy envi- energies but from the turbid nutrient- annual growth bands. Coral growth ronment required by the branching laden water of Florida Bay (Plate 1). bands are like tree rings and are vis- corals. Not all Holocene reefs were Reefs less than 2 ka are very thin, ible, just like bone structures in dental able to keep pace with sea-level rise, indicating that conditions for coral or medical x-rays, when the coral core however. Those that did not survive growth in more recent times were is into slabs and x-rayed. In a core died because sea level too quickly not as good as those in the earlier from a live coral, growth bands can be for them to maintain their ideal depth Holocene. In modern times as recently counted back from the surface begin- and they slipped below their thresh- as in the 1960s, Holocene reefs sup- ning with the year of coring to obtain old. Holocene corals also built mag- ported actively accreting corals to the age at the bottom. Some growth nificent spurs and grooves on wind- some degree. Very few such reefs are bands are wider than others and are ward sides of reefs. The spurs thrived alive today and those that are, are in called stress bands. They indicate a until the corals became overcrowded. very poor condition. period during deposition of the annual Constrained by position of overhead One of the best places to see band when water temperatures were sea level and presence of sand-filled inside a fossil reef is at the old unusually warm or cold. Some bands grooves at either side, the corals even- at Windley Key Fossil Reef State Park fluoresce under ultraviolet light in tually succumbed for lack of space. on Windley Key in the middle Keys response to an infusion of humic acids Few coral spurs are actively accreting (Plate 1). The coral species compris- that became bonded to the skeleton. today. ing the emergent Pleistocene reef are Fluorescent humic acids are a compo- C14 age dates from corals at the massive framework-building head nent of . Thus, presence of the outer-shelf margin show that corals that live in low-energy condi- a fluorescent coral band indicates a Holocene reefs began growing sooner tions. The corals in this reef indicate period of high freshwater influx onto off the lower Keys than off the upper that sea level was too high for a surf the reef during deposition of the band. Keys, a result of earlier shelf flooding zone to have existed where they grew. The study of annual coral growth because of lower bedrock elevation It is commonly believed that sea level bands is called sclerochronology. to the southwest. However, Holocene at that time, 125 ka, was 7.6 m higher reefs lived longer off the upper Keys than at present, and it is known that Records in Carbonate Sands than off the lower Keys, a result sea level has not been as high since Finally, even the coarse- and fine- of later flooding and protection from then. grained carbonate sands that line the adverse oceanic elements by the reef tract contain environmental sig- Pleistocene-reef islands at the margin. natures. Sands record geologic pro-

3 Annual Growth Bands in Seven Montastrea annularis 1955-56 at Hen and Chickens Reef

Sept. 1974 1969-70

1963-64

1957-58

normal annual band

1941-42

mm 1941-42 0 100 stress band Above: X-radiographs showing correlation of band- Left: X-radiograph of slabbed core from a species ing in seven different Montastrea annularis coral of head coral (Montastrea annularis) at Hen and heads at Hen and Chickens Reef. Four at left are Chickens Reef showing annual growth bands. from heads killed by cold in the winter of 1969-70. Note wide abnormal stress band deposited in the Three at right are from corals that were alive and coral skeleton as a response to unusually cold water healthy when sampled in September 1974. Topmost temperatures during the winter of 1941-42. The live right-hand stress bands joined by bold line correlate head coral was sampled in the spring after deposi- with time of demise of corals in left x-radiographs. tion of the 1955-56 annual band. See Plate 1 for loca- Stress band for 1941-42 in five left x-radiographs tion of Hen and Chickens Reef. Approximate vertical indicates that conditions were equally severe for distance between annual bands is 1 cm. those five corals. Other minor stress bands occur that are very faint or that do not correlate from coral cm = centimeters to coral. Normal annual bands are correlative among 2.54 cm = 1 inch all corals. cesses. For example, seismic profiles reef ridges during the repeated rises response to underlying bedrock topog- across a submerged off the in Pleistocene sea level. Other types raphy. In time, (5) they are swept northeastern part of the shelf margin of sedimentary processes are observed westward and off the ridge by the show linear low-elevation features on the roughly rectangular sand-cov- currents, as shown in seismic profiles believed to be incipient reefs that have ered Marquesas- ridge far- of westward-dipping bedforms at the been buried by sediment. Their burial ther west in the Gulf of Mexico. The west edge of the ridge. The Quick- indicates that prevailing energy there ridge is ~10 m higher in elevation sands are so named, even on navi- during the Pleistocene was onshore than the surrounding shelf and is gational charts, because the sands are as it is today and that during a low- thus isolated from sand importation. always in motion. stand in sea level, winds and waves Five processes occur on the ridge. (1) Sands also record the types of along that part of the terrace washed Sands are produced in place (geologi- in the reef tract, and in the sands landward. Along the southwest- cal source) by the dominant organ- case of corals, sand analysis reveals ern part of the terrace, however, essen- isms, several species of a calcareous whether they are healthy. Carbonate tially sediment-free backreef troughs green alga (biological origin). (2) The sediments in reef ecosystems consist behind immense linear reefs indicate sands are molded into prominent tidal of skeletal remains of the biota that longshore energy and sand transport. bars and sand waves that lie directly inhabit the reef. Thus, they contain It is just this longshore sand-transport on bedrock, which are evidence of the a record of change in biota through direction, parallel to the reefs and strength and direction of flow space (laterally) and time (vertically, arc-shaped margin, that kept young across the ridge. (3) The sands fill through sediment cores), and through incipient reefs there sediment free and shallow (~3 m) bedrock lows on the these biotic changes the sands reflect allowed corals to re-colonize the hard ridge and (4) accrete behind bedrock corresponding changes in the environ- reef surfaces and to build extensive highs such as at Halfmoon , a ment. Hence, sands in a reef ecosys-

4 favoring by the United States Government or any agency thereof. any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or the Open-File Report 00-164 Plate 1 24 25 30' sea level was about 7.6 m higher than present. the reef and margin. Both rock formations accumulated approximately 125 ka when sandbars. Called the Miami Limestone, bars formed more or less perpendicular to Limestone, the reef parallels shelf margin. The lower Keys consist of cemented middle Keys are composed of a long arc- shaped . Known as the Key Largo The Florida Keys are divided naturally by shape and composition. upper on the upper slope south of Marquesas Keys in 80 to 190 m water. sediment free. The thickest (40+ m) accumulation in the study area occurs seaward edge of the Pleistocene margin reef. The terrace outlier reefs are also submerged rock ledge bordering the south-southeast sides of Keys and at behind the Pleistocene margin reef. Sediment is virtually nonexistent along a to bedrock with a rod. The thickest shelf accumulations occur in backreef troughs bedrock depression that also contains ~3 m of sediment, determined by probing are formed in place. Note the Marquesas Keys a shallow, nearly circular current-swept, sediment-free on the east side indicate that ridge sediments on the Marquesas-Quicksands ridge. Deep water three sides of ridge and a of Hawk Channel (major bedrock depression between the Keys and shelf margin), m thick. Areas of thinnest coverage occur along the inner shelf, on landward side shows that most of the sediments on shelf and submerged terrace are ~3 shelf margin. A submerged terrace lies at the toe of this reef. In general, map part of map denotes bare rock on seaward side a Pleistocene reef that forms the (pale pinks) to thicker (dark reds) thickest (rust). Linear white area in seaward on the map represent different thicknesses ranging from bare rock (white) to thinnest and sediments that overlie Pleistocene bedrock along the shelf margin. Colors Marquesas Keys in the Gulf of Mexico, and a map thickness Holocene reefs Plate 1. ¡ ¡ Shoal Halfmoon Margin Shelf Rebecca Shoal Index map showing tip of South Florida and Sable, the Keys, 82 82¡ ¡ New Ground Shoal 30' 30' Key West Marquesas-Quicksands Ridge Ellis Rock Marquesas Keys 82¡ 82 ¡

Sand Key Reef Gulf of Mexico

Lower Key

Hawk

s

Channel South Florida mainland. densely populated Florida Keys and health, and safety impacts on the potentially very dangerous economic, ecosystem would have significant, and physically essential coral reef Any loss of the economically viable 81 81 the world wide web at: maps and through the USGS marine homepage on ways are through descriptive and interpretive impact on resources. Two of the most effective on Earth history and processes, human objective, and informative scientific knowledge The USGS serves the nation by providing basic, North ¡ ¡ 30' 30' http://marine.usgs.gov Cape Sable

Middle Key . 81 North American Stratigraphic Code. Reference therein to s 81 ¡ ¡ constitute or imply its endorsement, recommendation, Florida Bay Key Windley Lidz, B.H., (in prep.), Lidz, B.H., 2000, Lidz, B.H., 1997, The Everglades Plate. Geological Survey Open-File Report 00-046, 4 p., 1 Decline in a Modern Reef Ecosystem. Importance of in Understanding Biological Report 97-453, 4 p., 1 Plate. Continental U.S. Keys: Preserving the Largest Reef Ecosystem in and Eastern Dry Docks to Rebecca Shoal. (3) to Eastern Dry Docks, (2) Reef to Sombrero Reef, (1) Keys and Reef Tract: Key West to the Marquesas-Quicksands Ridge Vaca Key to West and Sombrero Reef North Key Largo to Vaca Key and The Elbow Hawk Chan

Upper Keys References Cited in Text: Bedrock Beneath Reefs: the Fragile Coral Reefs of the Florida n 01 20 15 10 5 0

el U.S. Geological Survey Open-File Surface Features of the Florida Key Largo Land Outlier Reefs Buried Reef Flats Paleoshorelines 1.0 - 1.5 0.5 - 1.0 0.0 - 0.5 1.5 - 2 80 80 3 - 4 2 - 3 ¡ Measured in Meters ¡ 30' 30' Scale = 1:650000 Hen & Chicken's Reef Miles U.S. 40 + 30 - 40 20 - 30 10 - 20 5 - 6 4 - 5 6 - 7 8 - 9 7 - 8 9 - 10 Margin Shelf 25 24 30' ¡ ¡ Aerial photograph showing tidal bars and sand waves near the west side of the Marquesas Keys. Tidal bars are 1 m high and are oriented north/south in response to strong north/south currents. Sand N waves on top of tidal bars are oriented perpendicu- lar to the bars and many sand waves are awash at spring low tide. Dark areas are grass and various species of a calcareous green alga. Distance across TIDAL photo is approximately 2.5 km.

TIDAL BAR km = kilometers m = meters TIDAL BAR

SAND WAVES

tem are bioindicators of the health of Parrotfish and boring sponges are the of evidence for numerous significant the reef community and the quality major bioeroders. The same studies past environmental changes. (1) Thick of environmental conditions. Plastic- showed an increase in grains of a cumulative Pleistocene and continuous impregnated thin sections of sediment calcareous green alga in areas off Holocene coral-growth records verify samples are used to identify and count the upper Keys. Proliferation of cal- previous existences of healthy reef grains under a microscope. careous algae (preservable as sand ecosystems and thus the optimal con- Sand-grain analyses conducted in grains) and fleshy red, blue, green, ditions under which they grew. (2) 1963 and in 1989 showed that a and brown algae (non-preservable) Dated mangrove peat found beneath significant increase in coral grains is also commensurate with increased offshore reefs documents the times occurred in surface sands in areas off nutrient supply and a declining eco- and locations of earlier, seaward, the middle and lower Keys over those system. Increases in both coral and shorelines. (3) Landward reef migra- 26 years. The increase correlates with algal sand grains and in observed tion is evidence of rising sea level. an observed increase in nutrient influx growth of fleshy algae correlate with (4) Shifts from head-coral zones to and decline in coral reef health in the a conceptual model that shows the branching-coral zones within a reef same areas and time. In other words, effects of increasing nutrients on are signs of changing conditions from much higher percentages of coral frag- organisms that comprise the bottom- quiet water to presence of surf and ments occur in areas where there are dwelling population in a reef eco- are thus evidence for flooding of once diseased and dead corals than in areas system and how that population will protective offshore reef islands. (5) where corals are healthy. The reason change. Is the nutrient influx natural, Annual stress bands in corals indicate is twofold: algae grow on dead coral or is it the result of human activities? periods of severe water-temperature surfaces, attracting grazers, and bio- The USGS studies described here pro- changes, and fluorescent bands verify of dead skeletal material— vide part of the scientific background weather variations that produced high yet two more processes indicated by needed before these questions can be freshwater influx to the reef. (6) the sands, these biologic—is associ- answered—an understanding of natu- Sands record geologic processes of ated with organisms that break down ral changes within past ecosystems. transport direction, scouring of back- dead coral. Fish bite off attached reef troughs, in-place formation, algae along with bits of skeletal coral, Summary strength and direction of currents, herbivorous sea urchins graze, and infilling of bedrock depressions, and sponges, algae, fungi, barnacles, and The geologic record of the Florida accretion behind bedrock highs, as bivalves bore into the coralline rock. Keys reef tract is a rich archive as biologic processes of grazing

5 WAYS IN WHICH MAPPED BEDROCK, SEDIMENT, AND SEAFLOOR INFORMATION BENEFIT THE ECOSYSTEM

Management Issues Ecosystem Issues

1. Understanding the direct relation between geologic framework and biologic 1. Mitigation of physical damages due to boat anchors, divers, or ship groundings and choice of the best and most effective type of restoration— resources, such as localization of biotic communities. by transplanting live corals directly onto elevated bare rock or onto prefabricated substrates placed in sandy areas. 2. Understanding the direct relation between geologic framework and physical processes, such as direction and rate of groundwater flow. 2. Mitigation of -disposal practices—by changing cesspools and shallow injection that empty into porous rock to wastewater-treatment 3. Understanding the direct relation between protective islands, open tidal plants and requiring holding tanks on boats passes, and sources and effects of deleterious waters on reef vitality. 3. Mitigation of abnormal Everglades drainage—by removing levees and 4. Understanding the direct relation between, and variety in, localized physical canals to restore natural water flow environments and biological communities. 4. Mitigation of runoff pollution—by reducing use of fertilizers and chemicals 5. Understanding the relation between dominant organisms and dominant in inland farmlands sediment components and how changes in dominant sediment components indicate changes in the biotic community. 5. Siting and choice of equipment for installing fixed navigational structures and lighthouses (i.e., in barren sand holes or on bare bedrock) 6. Integrating scientific knowledge with resource-stewardship practices to attain sustainable archeological, ecologic, and socioeconomic (tourism, 6. Siting of mooring-buoy anchorages (i.e., in barren sand holes surrounded commercial fisheries) systems by live-rock habitats)

7. Improving capabilities for restoration and preservation of each 7. Siting and choice of equipment for maintaining dredged channels and ecosystem component for stormwater drainage pipes (i.e., sand removal with minimal siltation and turbidity) 8. Predicting variability and evaluating effects of coastal processes and ecosystem response to natural (rise/fall of sea level) and anthropogenic 8. Siting of areas least susceptible to pollution and turbidity for transplanting (pollutants) influences corals and sea grasses, such as areas where sediment is thick or thin, sand or mud, or where the porous limestone is coated with caprock (i.e., 9. Educating the visiting public with accurate scientific knowledge of: thin sand or bare rock allow potentially contaminated ground waters to • how and why the many varieties of reef-tract resources exist reach surface waters and reefs; areas of thick sand or mud or caprock • how and why each type is an integral part of the ecosystem and form impervious aquicludes) must be preserved • how and why each can be damaged or destroyed and if destroyed, how an out-of-balance ecosystem will respond Systematic shelf-wide USGS mapping of sediment-constituent 10. Assessing impacts on the ecosystem before issuing treasure-hunting permits grains (Lidz, 1997), bedrock topography (Lidz, 2000), reef and sediment thickness (this report), and features and bottom environments (three reports, in prep.) aids both 11. Informing politicians, the media and public visually and descriptively of Sanctuary management-policy and ecosystem issues. locations and types of various habitats and environments prior to on-site visits for algae and resultant bioerosion of standing the evolutionary history and For more information, coral skeletons. (7) Sands are bio- characteristics of ancient reefs, the please contact: indicators of the types of reef organ- processes that produced or destroyed isms and the general health of corals them, and the responses they had Barbara H. Lidz through space and time. to environmental change. The reefs, U. S. Geological Survey 600 4th Street South These geologic signatures are corals, and carbonate sands are far St. Petersburg, FL 33701 guides to reef-ecosystem health and more important than simply being phone: 727-803-8747 x3031 environment and prove that conditions major components in a reef ecosys- fax: 727-803-2030 have been both good and bad for tem. Their history and what they can [email protected] reef ecosystems in the past. Is the tell us about past environmental con- fragile modern ecosystem undergoing ditions comprise one of the baselines This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards yet another natural decline, or are from which understanding the decline or with the North American Stratigraphic Code. Reference human activities responsible or, at of the modern reef system must be therein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise the very least, an added component? derived. does not necessarily constitute or imply its endorsement, rec- We cannot know without first under- ommendation, or favoring by the United States Government or any agency thereof.

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