Shells as Recorders of Environmental Change—A Study in Florida Bay Introduction change? (2) Does the temporal growth record coincide with seasonal variations in salinity, temperature, or some other environmental vari- The South Florida region is currently undergoing a massive restora- able? (3) Can the chemical variations of individual shell layers be tion effort guided by the Comprehensive Everglades Restoration Plan accurately measured at a microscopic scale? (4) Are the variations in (CERP) written by a multi-agency group of Federal, State and local gov- the trace-element concentration of the shell layers the response to a ernments. One of the primary goals of the CERP is to restore the quan- biological function of the that is independent of changes in the tity, quality, and timing of freshwater delivery throughout South Florida water mass, or are the variations representing changes in salinity and to a more natural state. The natural flow of freshwater in the South temperature while the shell is secreted? (5) If the trace-element varia- Florida ecosystem was altered throughout the 20th century by the con- tions of the shell correlate to salinity and temperature, then can these struction of a series of canals and water-control structures. These systems variations be calibrated and used as a proxy to represent changes in the affected the estuaries of South Florida, including Florida Bay (located water mass through time? within Everglades National Park), by altering the natural seasonal mixing For this study, the marine bivalve cancellata (commonly of fresh and saline waters. In addition, construction of the Flagler Rail- known as the Cross-barred ) was selected because it met these road and, later, U.S. Highway 1 through the Florida Keys restricted the important criteria necessary to serve as a good “data recorder” of salin- exchange of water between the Atlantic Ocean and Florida Bay. In order ity change: (1) the is widely distributed and locally abundant to establish realistic goals and performance measures for restoration, it throughout Florida Bay; (2) although it prefers marine conditions, it is is essential to first determine how the natural system functioned prior tolerant of a range of salinities from 20 to >40 parts per thousand (ppt); to significant human alteration. The goal of this research is to develop (3) it is comparatively long-lived (estimates of 5-7 years for the oldest a method to identify historical patterns of salinity change throughout specimens); and (4) its thick, ornamented shell facilitates specimen the bay, perhaps at the seasonal scale, from the remains of preparation, identification of seasonal growth, and accurate tracing of preserved in shallow-water sediment cores. growth layers for geochemical analysis. Setting Once the basic questions have been answered from studies of liv- ing Chione cancellata in both Florida Bay and the laboratory setting, Florida Bay (fig. 1) is a very shallow, subtropical estuary that is a calibration data set can be established that will be used to compare subdivided by a complex mosaic of mudbanks and mangrove keys the animal’s shell chemistry with the chemistry of the water in which (islands) that constrain the movement of tides and currents. Variations it grew. The calibration data can then be applied to the analysis of in salinity and temperature within the bay are largely controlled by shells taken from isotopically dated sediment cores collected in Florida the annual cycle of wet and dry seasons. The cooler winter months of Bay and other South Florida estuaries as part of the U.S. Geologi- November to April constitute the dry season (average daily precipi- cal Survey’s Ecosystem History projects. The results will provide the tation is approximately 0.075 inches). During the hot, wet summer regional and temporal patterns of change in salinity over the last 100 to months of May through October, precipitation increases three- to four- 500 years, which is necessary to set realistic and obtainable perfor- fold (average daily precipitation is 0.25 to 0.30 inches). mance measures for restoration. The natural precipitation cycle, though annually variable, drives the timing and amount of natural freshwater available to the South Florida Field Work ecosystem. Low regional rainfall during the winter reduces the freshwa- In February 2001, scientists placed two habitat enclosures in Whip- ter input, both as direct rainfall and as reduced runoff from the ter- ray Basin, an isolated basin in central Florida Bay (figs. 1, 2). The loca- restrial Everglades, into the bay. This decrease in freshwater raises the tion was established because of the close proximity to an Everglades salinity in the bay, especially along the northern margin where terrestrial National Park water monitoring station that records daily temperature, runoff from the Everglades enters the bay. During the wet season, the salinity, and precipitation. One hundred Chione cancellata specimens, abundance of regional rainfall, both as direct precipitation into the bay collected locally, were labeled, digitally photographed, and placed in and as increased outflow from the terrestrial Everglades, increases the each habitat. freshwater input to the bay and decreases the salinity. The result of these seasonal changes is that salinity can vary significantly in northern and Everglades National central Florida Bay. These changes create a rigorous environment in Park which only very adaptable organisms can survive. n Whipray Basin a e c Marine Animals as Data Recorders O c ti Gulf of n Information on the regional variation and seasonal changes in the Florida Bay a Mexico tl water chemistry of Florida Bay is reflected in the variations in the shell A chemistry (trace elements and stable isotopes) of marine organisms. Water Depth As the animals grow by adding sequential layers of calcium carbonate, < 3 feet FLORIDA they are recording information about the water in which they grow. By > 3 feet Study learning how to “read” the record contained in the shells, scientists can Area 0 5 10 Miles derive information about both modern and historical changes in salin- ity, temperature, and freshwater sources. Figure 1. Satellite image of southernmost Florida showing Whipray In order to interpret the shell chemistry data, the following ques- Basin, where two growth habitats were placed in order to study Chione tions must be answered: (1) What are the spatial and temporal patterns cancellata. Most of Florida Bay lies within Everglades National Park of the animal’s growth and how are these patterns related to seasonal (blue dashed line). Image from Jones and others (2001).

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P n=45 n=253 n=189 n=93 n=66 n=176 n=140 n=86 Since February 2001, the specimens have been retrieved on a regu- 0 Aug. Feb. May Aug. Oct. Feb. Aug. Dec. lar winter and summer schedule and have occasionally been retrieved 2001 2002 2002 2002 2002 2003 2003 2003 in the spring and fall to provide additional data. Each specimen, Habitat re-collection dates and number of individuals (n) whether dead or alive, was compared to its previous photograph to determine when and how much growth had occurred. All live speci- Figure 3. Histogram illustrating percentage of growth versus no growth mens that exhibited growth were digitally rephotographed prior to their in Chione cancellata populations in Whipray Basin habitats. The popu- replacement in the field habitat. Dead and lost specimens were replaced lation shows a response to the seasonal climate cycle. Between 40 with new labeled and photographed individuals to maintain a statisti- and 47 percent of the individuals retrieved during the summer showed cally significant population of approximately 100 individuals. measurable growth, compared to 1 to 12 percent during the fall and winter, with the exception of February 2002. When do Chione cancellata grow? Preliminary data from field growth studies indicate that Chione cancel- 800,000 Original specimen size lata from Florida Bay can provide the necessary seasonal growth record New growth measured )

over time for shell-chemistry analysis. Data analyzed over three years s l after six months e 600,000 x indicate that the habitat population grew in response to the seasonal water i p n temperature cycle (fig. 3). The preliminary data indicate that during those i ( n months when the water mass is cool to cold (fall, winter, and early spring), e m

i 400,000 over 80 percent (typically over 95 percent) of the population did not exhibit c e p growth; however, data from the late spring and summer indicate that 40 s f o

to 47 percent of the individuals had measurable valve growth. During e z

i 200,000 this period, both water temperature and salinity increased from the winter S minimums to late spring and summer maximums. Data also indicated that juvenile and young adult individuals repre- 0 sented the largest part of the population that exhibited growth (fig. 4). 1 5 10 15 20 25 30 35 40 Image analysis of digital photographs of the specimens with confirmed Number of specimens measured growth indicated that juveniles accounted for 25 percent of the popula- tion and young adults accounted for 20 percent of the population; Figure 4. Histogram showing the preliminary growth analysis of Chione however, “old-age” individuals accounted for less than 1 percent of cancellata at Whipray Basin from February 2001 to August 2001. All this population. In fact, some “old-age” specimens in the study did not specimens were digitally photographed at the same distance and magni- grow for 2 to 3 years. In general, small specimens showed more con- fication. The histogram shows the size distribution in pixels (determined tinuous, sequential annual growth, while larger individuals grew more from image-processing techniques) of 40 specimens photographed both episodically in 2- to 3-year increments. before and after six months. New shell-margin growth principally occurs These combined data indicate that as Florida Bay water temperatures in smaller juvenile to young adult individuals, while very little growth is and salinities increase from winter minimums to late spring and summer observed in the larger, mature to “old-age” individuals. maximums (fig. 5B), 40 to 50 percent of the Chione cancellata population of Florida Bay show growth. Of this population, a majority of juvenile of shell growth. The outer prismatic layers show successive cycles and sub-adult individuals will secrete biogenic calcium carbonate, which of growth in the form of groupings of laminae (fig. 6). These groups records the physical and chemical changes in the water mass. of laminae are set apart by both distinct and subtle punctuations, or “breaks,” that represent cessation of calcium-carbonate secretion; there- How do Chione cancellata grow? fore, a missing period of time in the shell’s “data recorder” is created. Chione cancellata have a morphology similar to many other mol- Three different scales of cyclical groupings, or bundles of laminae, can luscan bivalves. The first and most obvious common feature is the be detected that probably represent daily, intraseasonal, and seasonal two-part valve structure, which is characterized by a thin, relatively growth history. featureless inner layer of aragonite, and a thicker, outer prismatic layer of subparallel, anastomosing calcite crystals (fig. 6). The second com- How do scientists detect change in shell layers? mon feature, which is more difficult to detect, is the presence of paral- Microbeam instruments, such as the electron microprobe (EM), are lel laminae secreted at the shell margin, which represents incremental designed to measure the composition of extremely small amounts of growth. The laminae are approximately 1 μm (micrometer) thick in the material. The small probe diameter (10 μm) and close spatial sam- inner aragonite layer and thicken to 5 to 7 μm in the outer calcite layer. pling resolution (5-50 μm) of these instruments make them ideal for Throughout the Chione cancellata’s life history, changes in the measuring the compositional changes at or near the micrometer-scale physical environment (seasonal temperature, salinity, food supply, and thickness of the shell-growth laminae. The EM performs both quantita- turbidity), together with the individual’s age, affect the timing and rate tive and semiquantitative analyses by detecting characteristic X-rays through sequential growthrecordofthespecimen. TheMg/Cacounts, spacing provides ahigh-resolutionrecordoftrace-element variation showing valvemorphology andEMlinetransects. The10- calcium. of magnesiumtocalcium;lower graph plotstheratioofstrontiumto (EM) trace-elementanalysisofthis specimen;topgraphplotsratio temperature andsalinityfordates ofgrowth. 2002. showing growthrecordofthevalvefromFebruary3,2001,toMay 2, Figure 5. C D A B t G o r , GraphshowingWhiprayBasinwater-monitoring recordof o Mg/Ca 5 w Sr/Ca /

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2 T r A A a a 3 o m m / g m 0 0 6 C 3 3 w . p p / 6 6 6 l l C e e . . a a 6 6 s s r a # d t 2 2 W W W o t G 7 7 u o t r R R 6 6 t o o s 2 s m R w / A A a a a 7 W m / t t m 0 0 0 A h b l a 2 3 3 t G Chione cancellata p 8 R o p 6 6 o r 0 / l c o 2 l l 8 e . . A e 0 5 5 w / 3 2 s o / s c 0 0 0 t h 6 1 / 3 0 u o 2 1 W 6 / 3 n u . / 0 5 R 1 t n mm A s 2 t 0 2 s / 3 C 0 / 6 , Electron-microprobe 3 1 1 0 o B . W W 7 7 / 5 5 4 Temperature, C 3 7 7 0 R R 1 / s s A A 0 a a Salinity, ppt 4 5 1 2 3 0 0 specimen#WRA036 m m 2 1 0 0 0 0 0 3 3 / p p 6 6 0 l l . . e e 4 4 3 s s J / a 0 n μ 1 m sample F W e b R A D 4 4 M 0 i a s 1 1 3 r W W 3 3 t 6 2 2 a R R . n s s 3 A A A a a c p 0 0 m m r e 3 3 p p 6 6 8 l l . . f e e 3 3 r M / of 2002(February toMay),trace-elementconcentration alsoincreases. temperatures and salinityagainincreasethrough latewinterandspring variation andshell morphologyshowlittlevariability. However, aswater temperatures andsalinityaredeclining tominimums,trace-element Between August2001andFebruary 2002(fallandwinter),whenwater moves fromwinterminimumsto late springandsummermaximums. intraseasonal watertemperature andsalinitycyclicityaswatermass shell growthandcyclicalvariability ofMg/Caappeartocorrespond related toshellmorphology. FromFebruarytoAugust2001, bothepisodic and toalesserextentSr/Ca,showsignificantvariationsinconcentration s s 2 o a 0 y m / 0 8 J m 1 u / n 2 a e 3 3 r 0 8 g W / / J 2 i 0 u n l 0 R 1 y / ( A 0 1 1 m 1 W W 7 7 0 A 8 8 m u 3 R R g s s 6 W A A a a ) . 0 0 m m 2 3 3 h S p p 6 6 e l l . . i p e e 2 2 p t s s r a 2 2 O y E c t v B e a N r W s o g 2 v i / l n R T S 2 0 a e a 0 A 2 7 d m D w l 0 i / / 0 e n 0 e 0 p c 1 3 a i 2 2 s e t 7 6 t y / r 2 e / 0 . a N J ( 1 0 0 t p 7 r a u n 2 a 0 p / m r 0 1 1 t 2 t e ) 2 i o ( o F 1 1 b e W W 6 6 n n o b 7 7 R R t a i t s s t A A o l a a o 0 0 M m m m P 3 3 r a p p 6 6 , i a r l l . . n M o e e 1 1 r C s s g k 5 a ) A 5 p r / s r 0 5 g / t 0 / i 2 a 0 n 2 t / M 0 0 2 / 0 i o 0 a o / y 0 2 2 f n 2 s J h u n e e l l Toward umbo Toward margin summer seasons, which also is the period when salinity and tem- perature increase from winter minimums to summer maximums. 3. Juvenile and sub-adult Chione cancellata generally exhibit both Prismatic the greatest amount and the most continuous rates of shell growth. calcite Outer layer layer 4. Sequentially secreted growth laminae of the Chione cancellata shell are punctuated by discontinuities due to cessation in growth during seasonal and intraseasonal cool to cold water temperatures, Nacreous which means information is missing from the shell “data recorder.” Inner layer aragonite 5. High resolution electron-microprobe analyses of Chione cancel- layer lata shells, which have documented growth during specific time 100 µm periods, produce accurate “maps” of changes in trace-element Specimen concentrations over a given period of time. WR181 6. The highly variable ratios of magnesium to calcium, and more Inter-annual growth band subtly of strontium to calcium, appear to record both a first- and 100 µm a second-order temporal cycle of seasonal and intraseasonal changes in water mass related to salinity and (or) temperature. Figure 6. Scanning-electron microscope photograph of a cross section of an etched Chione cancellata valve (specimen WR181) illustrating the Will shell analyses be a useful tool for restoration? two-part calcium-carbonate shell structure and the more subtle growth Although additional investigation is needed, shell-chemistry laminae sequentially secreted through time. In the outer layer, wavy analyses of Chione cancellata can be used to extract information about calcite prisms are secreted perpendicular to parallel, nominal ~5-μm- its environment; however, several cautions are in order. First, Chione thick growth laminae; the laminae form 10- to 20-μm-thick couplets, cancellata do not grow continuously throughout the year; therefore, which in turn form variably thick, light to dark, 60- to 120-μm-thick inter- although they are not recording a complete annual history, the prelimi- annual growth bands. nary studies indicate that at least 50 percent will show growth during the summer maximums in salinity and temperature. Second, if only emitted from selected elements of interest in the shell material. As the 50 percent show growth at any one time, either the results will need concentration of an element increases in the shell layer, a proportional to be statistically averaged when analyzing specimens from cores, or number (or count) of X-rays also increases. some other method of dealing with the potential lack of growth during The middle and lowest graphs on figure 5 illustrate the results of six EM a given time period must be determined. Third, analyses should be line analyses of a highly polished longitudinal section of Chione cancellata limited to juveniles and sub-adult individuals, or to the juvenile and specimen #WRA036 (fig. 5A) from Whipray Basin. The transects, which sub-adult portions of the shells of the older specimens. parallel the axis of growth, have a spatial sampling resolution of 10 μm and Whether temperature or salinity is the controlling factor on trace- a 10-μm probe diameter, which provides 100 compositional analyses per element uptake is a question that still needs to be answered; however, millimeter. This sampling resolution allows scientists to derive seasonal and for South Florida, this issue is not critical because increases and intraseasonal patterns of compositional change from the shell. decreases in temperature and salinity generally parallel each other and What do scientists measure in the shell? this relationship can be demonstrated statistically. Hence, the cau- tious analysis of Chione cancellata specimens from isotopically dated All living organisms either actively or passively sequester trace sediment cores from Florida Bay and other South Florida estuaries will elements from their environment. Marine organisms, like Chione can- provide information about historical changes in the environment. The cellata, incorporate trace elements from the water in which they live results of this study potentially could be applied to other species of into the calcium-carbonate structure of the shell. Magnesium (Mg) and Chione and possibly to other genera of the family. strontium (Sr) have many physical properties in common with calcium (Ca), and thus can easily substitute for calcium in the shell structure. By Robert G. Stamm and G. Lynn Wingard The concentration of magnesium and strontium in the world’s oceans has remained stable for thousands of years. Both elements also are stable relative to the calcium that is a component of sea salt (CaCl); therefore, References Cited as the amount of salt in the water varies, the amount of magnesium and strontium varies. Previous studies (Dwyer and Cronin, 2001, and refer- Dwyer, G.S., and Cronin, T.M., 2001, Ostracode shell chemistry as a ences therein) have demonstrated that the ratios of Mg/Ca and Sr/Ca in paleosalinity proxy in Florida Bay, in Wardlaw, B.R., ed., Paleo- organisms with calcium-carbonate shells can be used to calculate the ecologic studies of Florida Bay: Bulletin of American Paleontology salinity of the water in which they grew. In this study, those ratios were 361, p. 249–276. measured in the EM transects across the shell as shown in figure 5C. Jones, J.W., Thomas, Jean-Claude, and Desmond, G.B., 2001, South Considerable questions still exist about the effect of water temperature Florida Everglades: U.S. Geological Survey Miscellaneous Investiga- and biological processes on the uptake and secretion of magnesium, tions Series Map I–2742, scale 1:100,000, 2 sheets. [Also available calcium, and other elements. These questions still need to be answered. online at http://sofia.usgs.gov/projects/remote_sens/sflsamap.html] What do we know so far? For further information, please contact: The following points summarize the results to date: Robert G. Stamm G. Lynn Wingard 1. Marine organisms (such as Chione cancellata) that secrete U.S. Geological Survey U.S. Geological Survey biogenic calcium-carbonate record information about the water in 926A National Center 926A National Center which they live. Reston, VA 20192 Reston, VA 20192 2. Close to 50 percent of the Chione cancellata population of Telephone: 703-648-5271 Telephone: 703-648-5352 Florida Bay exhibits measurable growth during the spring and email: [email protected] email: [email protected]