Highly variable sediment distribution in shallow, wind ... - Springer Link

Journal of Paleolimnology 15: 20%221, 1996. @ 1996KluwerAcademicPublishers. Printedin Belgium.

207

Highly variable sediment distribution in shallow, wind-stressed lakes: a case for s e d i m e n t - m a p p i n g surveys in paleolimnological studies * Thomas J. Whitmore, Mark Brenner & C. L. Schelske Department of Fisheries and Aquatic Sciences, University of Florida, 7922 NW 71st Street, Gainesville, FL 32653, USA Received 10 August 1994;accepted20 October1995

Key words: sediments, paleolimnology, sediment distribution, sediment mapping

Abstract

We examine sediment distribution patterns in seven Florida lakes and discuss implications for paleolimnological studies of shallow, subtropical lakes. The study lakes are highly productive and should exhibit thick organic sediment deposits, but organic sediments are often grossly lacking because basins are shallow, and frequent mixing, lack of stratification, and warm temperatures lead to breakdown of organic material. Organic sediment distribution patterns are highly variable. We observe three types of distribution patterns. When organic sediments are abundant, there may be (1) uniform sediment distribution. In lakes lacking organic sediments, there are (2) distribution to deeper areas if present, or (3) distribution to peripheral areas and embayments when deep waters are absent. We advocate the use of systematic mapping surveys to locate optimal coring sites for paleolimnological studies of shallow, wind-stressed lakes. Because numerous factors affect diagenesis and sediment redistribution, sediment abundance and location are not predictable. Sediment chronologies may be discontinuous and disturbed even in accumulation zones. The extent to which sedimentary records are discontinuous or disturbed is not quantifiable in any practical manner. 21~ and 137Cs radioisotopic profiles provide qualitative evidence of the degree of stratigraphic disturbance. Total excess 2wPb inventories show that sediments are focused into depositional zones when sediment distribution is uneven. Excess 21~ inventories are not informative about the completeness of sedimentary profiles unless small inventories suggest discontinuous sedimentation or erosional events. We present examples o f disturbed and undisturbed profiles, and discuss how we use radioisotopic and geochemical evidence, and multiple cores to assess stratigraphic continuity.

Introduction

Our paleolimnological studies in Florida have revealed an unexpected aspect of sedimentation in Florida's shallow, subtropical lakes: many of these lakes show highly uneven organic sediment distribution, and organic sediments are almost entirely lacking in some hypereutrophic lakes. Sediment distribution reflects dramatically distinct erosional and depositional zones, and appears to be affected by morphometry as well as prevailing winds that cause resuspension and redistribution of sediments. It is especially problematic under

* Journal SeriesNo. R-04815 of the FloridaAgriculturalExperiment Station.

these conditions, therefore, to select coring sites that contain long records of continual deposition. Several paleolimnological studies have addressed variation in sediment distribution within lake basins. Anderson (1989, 1990a, 1990b), for instance, examined spatial variability in accumulation rates of dry mass and diatoms in numerous cores from Lough Augher, Northern Ireland. Dry mass accumulation rates were variable and not correlated with water depth, and no single core accurately reflected mean accumulation rates for the entire basin. Odgaard (1993) studied sediment yield and sediment distribution patterns for the past 10000 years in Sk~nsr a small kettle lake in northwestern Denmark. Organic sediment distribution in Sk~nsO is significantly affect-

208 ed by strong winds that cause sediments in shallow areas to be resuspended and transported downwind to deeper water where they are deposited. Of 36 sediment cores studied, none contained organic sediment at basin depths of less than 1.6 m. Loch Fleet, a small (17.1 ha), wind-stressed lake in southwest Scotland, contains maximum organic sediment thickness in 8 m of water on the leeward side (Anderson & Battarbee, 1985), whereas sediment accumulation in the deepest (16.5 m) part is minimal and discontinuous (Anderson et al., 1986). Wind-stressed effects on organic sediment distribution have also been observed in lakes of England and North Dakota, USA (S. Fritz pers. comm.). Most lakes in which variable sediment distribution patterns have been studied are cold temperate, and a systematic approach to locating appropriate coring sites in lakes with spatial variability in sediment distribution has not yet been proposed. Sediment focusing can be assessed by examining total inventories of radionuclides that are deposited in lake sediments from atmospheric sources (Crusius & Anderson, 1995). Turekian et aI. (1977) estimate a global average of 21~ atmospheric fallout of approximately 0.5 pCi cm -2 yr -I (1.1 dpm cm -2 yr-1). This value has been confirmed for Florida lakes by measurement of excess Zl~ inventories in cores from 9 lakes (Binford & Brenner, 1986). An atmospheric fallout rate. of 1.1 dpm cm -2 yr -1 for 21~ would produce a steady-state inventor3, for excess 21~ of approximately 32.5 dpm cm -2. This predicted inventory value provides a standard of comparison that is useful for determining whether sediment focusing has occurred at a specific site. Total excess 21~ inventories that are larger than the predicted value indicate an additional source of deposition, such as redistribution of sediment from other areas within the basin. Total inventories smaller than anticipated suggest sediment removal from a site. In this paper, we document sediment distribution patterns in seven, shallow, subtropical lakes in central Florida. We discuss how sediment mapping surveys help us to locate coring sites in depositional zones that are least subject to stratigraphic disturbance. We also discuss ways to assess the stratigraphic integrity of sediment cores using radioisotopic (21~ and 137Cs) and geochemical evidence. Study lakes

Lake Maggiore is a small (147 ha), shallow (z,~a~ = 3.0 m), hypereutrophic (CH2M Hill 1992) lake

in a residential area of St. Petersburg, Pinellas County, Florida (27 ~ 44 t N, 82 ~ 39 r W). Its watershed covers 1350 ha and is underlain by Pleistocene deposits of the Fort Thompson Group (Brooks, 1981). Surface water enters via a stream in the northwest corner of the lake and discharges to Tampa Bay via Salt Creek at the northeast corner. Lake hydrology was altered in 1927 when Salt Creek and the northern edge of the lake were channelized which allowed salt water to enter the lake. A weir was constructed on the creek in 1941 to control lake level and limit saltwater intrusion. Lake Hollingsworth is a small (144 ha), shallow (zm~ =2.15 m, z ~ = 1.5 m), urban lake in Lakeland, Polk Co., Florida (28 ~ 0t r N, 8t ~ 56 ~ W). The basin lies in the Bartow Embayment division of the Central Lakes District, where underlying bedrock is comprised of phosphatic sands and clays of the Bone Valley Formation. Water quality information indicates that the lake has been hypereutrophic for at least 25 years (Huber etaL, 1982, Canfield & Hoyer 1992). Clear Lake is a small (64 ha), eutrophic (Canfield & Hoyer, 1992) lake in the Dade City Hills division of the Ocala Uplift District (Brooks, 1981) in Pasco County, Florida (28 ~ 20 r N, 82 ~ 15 ~W). Local geology is dominated by sand, silty sand, and clays of the Hawthorne Formation. The lake has a z ~ of 8.2 m (Leslie et al., 1983) and a z ~ e ~ of 5.9 m (Canfield & Hoyer, 1992). The Clear Lake watershed (233 ha: Leslie et aI., t983) contains low-density residential development, a small college, and agriculture. Lake Thonotosassa is a shallow (z,~a~ =4.3 m), hypereutrophic (Southwest Florida Water Management District 1995) lake in the Gulf Coastal Lowlands physiographic region of Hillsborough County (28 ~ 03 ~ N, 82 ~ 16~ W). The lake has a surface area of 332 ha and a relatively large watershed (2330 ha). Surface flow enters through Baker Creek, the only inlet stream, and exits via Flint Creek in the southeast corner. In recent decades, the lake received agricultural and urban stormwater runoff, and effluent from the Plant City municipal wastewater treatment facility and food-processing plants (Cowell et al., 1975). Lake Marianna is a small (204 ha), shallow (z,,~,~ = 3.8 m: Canfield & Hoyer, 1992) lake located north of Winter Haven in Polk County, Florida (28 ~ 04 p N, 81 ~ 451 W). Lake Marianna lies in the Winter Haven Karst division of the Central Lake District, and is underlain by Miocene and Pliocene phosphatic sands and clays of the Bone Valley and Hawthorne Formations (Brooks, 1981). The lake is currently alkaline and eutrophic (Canfield & Hoyer, 1992).

209 Lake Parker is a moderately large (920 ha), hypereutrophic, shallow ( z m ~ = 3 m) lake in Polk County, Florida (28 ~ 04' N, 81 ~ 55' W) that lies adjacent to the city of Lakeland. The basin is in the Polk Uplands physiographic region and is underlain by phosphatic deposits of the Hawthorne and Bone Valley Formations (Canfield, 1981). Lake Seminole, in Pinellas County (27 ~ 50' N, 82 ~ 47' W), is a small (277 ha), eutrophic lake (Canfield 1981, Southwest Florida Water Management District 1992) in the Gulf Coastal Lowlands physiographic region of Pinellas County. The lake is underlain by phosphatic deposits of the Hawthorne Formation (Puri & Vernon, 1964). Prior to the 1940s, the lake was part of Long Bayou, a tidal estuary of Boca Ciega Bay. In the 1940s, a weir constructed at the south side prevented further tidal flushing and estuarine influence. Freshwater pumped into the lake over a second weir constructed at the north end reduced hydraulic residence time and led to the establishment of Lake Seminole as a freshwater ecosystem (Southwest Florida Water Management District, 1992).

Methods We conducted sediment mapping surveys that allowed us to assess the areal distribution, thickness, and organic-matter content of soft sediments throughout each lake basin. First, we laid a grid over the bathymetric map of each lake and selected sampling sites that were located to achieve approximate equal area coverage of each basin (H~kanson, 1981). Target sampling sites were located along a series of equally spaced north-south or east-west transects. In the field, starting points of the transects were located using shoreline features recognizable on maps. Destination points at the end of each transect were sighted with a Suunto compass, and travel time between shores was measured while running our boat at a slow, constant speed. Sampling locations were determined by dividing each transect into segments of equal running time during our return to the point of origin. At each station we recorded compass bearings to visible shoreline features, such as water towers or buildings. Station locations on Lakes Marianna and Thonotosassa were also recorded with a Trimble Navigation Pathfinder Global Positioning System. At Lake Maggiore, sampling stations were located using a LASERTRAK Range/Azimuth system that consists of a surveyor's theodolite attached to a precision rang-

ing laser that was interfaced to a radio telemetry system. The LASERTRACK was operated from a known benchmark station on the east shore. We measured soft sediment thickness at each station by forcing metered metal rods (electrical conduit or magnesium-zirconium coring rods) through the sediment lens until contact was made with underlying sands or clays. Water depth was measured by lowering a Secchi disk on a metered rope to the sediment surface. Soft sediment thickness was calculated by subtracting water depth from the total depth to hard bottom. Once soft sediment depth was determined, a sediment/water interface core was collected at each site using a 4-cm diameter cellulose-butyrate piston corer. We measured, photographed, and described the stratigraphy of each retrieved core. Surface sediments were collected with an Ekman dredge at 18 stations in Lake Seminole for gravimetric analyses. Sediment density (g dry cm -3 wet sediment) was measured by weighing volumetric samples that were dried for 48 hr at 80 ~ Organic matter content of these samples was measured by loss on ignition at 550 ~ for two hours in a muffle furnace (Hgtkanson & Jansson, 1983). Particle-size analysis was performed using a LaMotte Chemical Soil Texture Unit following dispersion with a 2% solution of sodium pyrophosphate (Foth, 1970). Size fractions were then dried at 70 ~ and combusted at 550 ~ to remove organic material. Following combustion, the sand fraction was further subdivided by sieving through a 250 #m sieve. All size fractions were weighed in order to express particle-size composition on a percent basis by weight. Sediment cores were collected from two sites in Lakes Hollingsworth, Thonotosassa, Marianna, Parker, and Seminole, and from one site in Clear Lake. We present radioisotopic and sediment chemistry profiles from three lakes to evaluate stratigraphic correlation at distant sites in a lake with even sediment distribution (Hollingsworth), in a lake with sediments distributed to deeper areas of the basin (Marianna), and in a lake with sediments distributed to peripheral areas (Seminole). Several criteria were used to pick coring locations. First, we considered sediment thickness data and stratigraphic descriptions of preliminary cores taken at each site. We eliminated stations possessing less than 1 m of organic sediment to avoid retrieving truncated sediment records. We selected stations that were sufficiently distant from shore to minimize littoral effects and were distant enough from each other to permit stratigraphic correlation from different portions of the

210 basin. In all cases, stations were located where water depth and soft sediment thickness suggested that a continual depositional history was likely. Sediment cores were collected using a piston corer designed to take undisturbed sediment-water interface sections (Fisher et al., 1992). The cores were extruded in a vertical position and sectioned at 2-cm intervals. Stratigraphic disturbance and correlation between sites were evaluated using radioisotopic and nutrient content profiles. Samples for radiometric dating were dried and ground to fine powder. Samples were sealed with epoxy glue in plastic Sarstedt tubes for at least three weeks before counting to trap 222Rn gas produced by in situ radium, and to establish equilibrium between 226Ra and 214Bi. 226Ra values are estimated directly from 214Bi activities. 21~ 214Bi, and 137Cs radioisotopic activities were assayed by gamma counting (Schelske et aL, 1994) using an ORTEC Intrinsic Germanium Detector connected to a 4096 channel, multichannel analyzer. Excess 21~ the unsupported atmospheric component of total 21~ activity, was calculated by difference between total 21~ and 226Ra activities. Total excess 21~ inventories of sediment cores were calculated by summing the products of excess 21~ activity (dpm g - l ) and bulk density values (g dry cm -3 wet material) for each sample in the cores. In order to estimate total excess Zl~ activities for the Lake Parker sediment cores, activities for every other sample in the cores were approximated by linear interpolation. Subsamples for nutrient content analyses were dried and ground to a fine powder using a mortar and pestle. Total N in the Lake Seminole core was measured by autoanalyzer (US EPA 1979) following a modified Kjeldahl digestion (Bremner & Mulvaney 1982), and in the Lake Hollingsworth and Marianna cores with a Carlo Erba NA 1500 Carbon/Nitrogen/Sulfur Analyzer. Total P in the Lake Seminole core was measured by autoanalyzer following the modified Kjeldahl digestion. In the Lake Hollingsworth and Marianna sediment cores, total P was measured using a Bran + Luebbe Autoanalyzer II with a single-channel colorimeter following digestion with H2SO4 and K~_S2Os (Schelske et aI., 1986).

Results Three patterns of organic sediment distribution were observed in the study lake basins. Organic sediment distribution was relatively uniform in Lakes Maggiore

and Hollingsworth. Organic sediments were scarce over much of the basin areas in Lakes Clear, Thonotosassa, and Marianna, but were focused in deeper portions of these lakes. Lakes Parker and Seminole also demonstrated scarce organic sediments, but their basins lacked deep areas. Organic sediments in Lakes Parker and Seminole were redistributed by wind-generated currents to embayments and peripheral areas of the basins. Lakes with uniform sediment distribution

A survey of 30 stations in Lake Maggiore demonstrated that sediment thickness was fairly uniform throughout the basin. Water depth was generally _75% water content is found at 9 out of 11 sites where soft sediment accumulation is >_0.5 m in thickness, and