Thomas J. Whitmore

Stable isotope (δ13C and δ15N) signatures of sedimented organic matter as indicators of historic lake trophic state

We explored the use of carbon and nitrogen isotopes (δ13C and δ15N) in sedimented organic matter (OM) as proxy indicators of trophic state change in Florida lakes. Stable isotope data from four 210Pb-dated sediment cores were compared stratigraphically with established proxies for historical trophic state (diatom-inferred limnetic total phosphorus, sediment C/N ratio) and indicators of cultural disturbance (sediment total P and 226Ra activity). Diatom-based limnetic total P inferences indicate a transition from oligo-mesotrophy to meso-eutrophy in Clear Lake, and from eutrophy to hypereutrophy in Lakes Parker, Hollingsworth and Griffin. In cores from all four lakes, the carbon isotopic signature of accumulated OM generally tracks trophic state inferences and cultural impact assessments based on other variables. Oldest sediments in the records yield lower diatom-inferred total limnetic P concentrations and display relatively low δ13C values. In the Clear, Hollingsworth and Parker records, diatom-inferred nutrient concentrations increase after ca. AD 1900, and are associated stratigraphically with higher δ13C values in sediment OM. In the Lake Griffin core, both proxies display slight increases before ~1900, but highest values occur over the last ~100 years. As Lakes Clear, Hollingsworth and Parker became increasingly nutrient-enriched over the past century, the δ15N of sedimented organic matter decreased. This reflects, in part, the increasing relative contribution of nitrogen-fixing cyanobacteria to sedimented organic matter as primary productivity increased in these waterbodies. The Lake Griffin core displays a narrow range of both δ13C and δ15N values. Despite the complexity of carbon and nitrogen cycles in lakes, stratigraphic agreement between diatom-inferred changes in limnetic total P and the stable isotope signatures of sedimented OM suggests that δ13C and δ15N reflect shifts in historic lake trophic state.

Ecosystems, paleoecology and human disturbance in subtropical and tropical america

Abstract Human disturbances of ecosystems last a long time and have quantifiable influences on the structure and function of the systems. If long records (e.g. paleoecological) of both disturbances and the responses are available, the array of disturbances provides quasi-experimental treatments useful for the study of factors which govern ecosystems. This paper examines the paleoecology of a series of lake-drainage basin ecosystems that have been subject to disturbances which vary through time and space. In all cases studied, it has been demonstrated that human activities have increased the movement of materials from the catchment to the lake. Examples in Guatemala, Haiti and Florida demonstrate that the flow of macronutrients (nitrogen and phosphorus) is proportional to human population sizes, and that the flow of inorganic particulates is related to the nature of both the disturbances and the catchment. Lake eutrophication is driven by growing human populations, but the rate of increase can be slowed by activities such as urbanization, which increases siltation. Several tropical ecosystems have recovered from severe disturbances, but the rate of recovery was related to the severity and temporal extent of the disturbances.

Holocene climatic and human influences on lakes of the Yucatan Peninsula, Mexico: an interdisciplinary, palaeolimnological approach

We used palaeolimnological techniques to examine effects of Holocene climate change and human influence on lakes of the Yucatan Peninsula, Mexico. The three study lakes are located along a west-east transect that represents a gradient of increasing modern precipitation and density of former Maya settlement. At Lake Coba, an 880-cm sediment core yields a complete record of lacustrine sedimentation that began when the lake first filled ∼8000 BP as groundwater level rose in response to rising sea level and increased precipi tation. Diatom, ostracod, and δ 18O evidence indicate that Lake Coba was initially shallow and saline. Coba, presently in the region of greatest rainfall, showed more episodic water-level changes than Lake San Jose Chulchaca or Lake Sayaucil. High lake level and fresh water were evident at 440 cm (∼2600 BP), followed by a decline in water levels and an increase in total ionic salinity to the present time. In a 613-cm core from Sayaucil, in the intermediate precipitation zone, total salinity was high between 600 and 400 cm (∼3050 and 2000 BP), followed by consistently higher water levels. Salinity was high in the lower portion of a 110-cm San Jose Chulchaca core (beginning ∼1860 BP), followed by a gradual and consistent freshening of water to the present time. Trophic state changes and human influence on lakes were evaluated using diatom, δ13C, total P, sedimentary organic matter, and preliminary pollen data. Maximal human disturbance at Lake Coba, a densely settled Maya urban site, occurred during a deep-water event at 440 cm, followed by a decline in human influence and trophic state to the present time. Trophic state and linear sedimentation rates in Sayaucil increased significantly above 400 cm (after ∼2000 BP), probably associated with initial Maya settlement near Xtojil and subsequent small- scale farming. Limnological disturbance may have preceded the period of maximal human occupation because initial land clearance and consequent soil erosion probably affected water quality substantially. San Jose Chul chaca lacks archaeological evidence of human occupation in the drainage, and shows gradual changes in trophic state not caused by human disturbance. With the exception of a 14C date on wood from the base of the Coba core, 14C dates and chronologies may be artificially old as a consequence of hard-water-lake error. Trophic state changes in the study lakes were generally consistent with known patterns of human settlement and popu lation change. Late-Holocene water-level fluctuations were most pronounced in Lake Coba in eastern Yucatan, where modern rainfall is currently greatest, but lake level is generally lower than during much of the past. Lake level was relatively constant in Sayaucil in the central peninsula, whereas lake level in San Jose Chulchaca in the arid western portion of the peninsula increased gradually over time.

Highly variable sediment distribution in shallow, wind-stressed lakes: a case for sediment-mapping surveys in paleolimnological studies

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. 210Pb and 137Cs radioisotopic profiles provide qualitative evidence of the degree of stratigraphic disturbance. Total excess 210Pb inventories show that sediments are focused into depositional zones when sediment distribution is uneven. Excess 210Pb inventories are not informative about the completeness of sedimentary profiles unless small inventories suggest discontinuous sedimentation or erosional events. We present examples of disturbed and undisturbed profiles, and discuss how we use radioisotopic and geochemical evidence, and multiple cores to assess stratigraphic continuity.

Paleolimnological Methods for Defining Target Conditions in Lake Restoration: Florida Case Studies

ABSTRACT Paleolimnological methods can be used to identify baseline limnological conditions and to assess anthropogenic changes in lakes that lack historical limnological data. We studied a suite of Florida lakes and developed regression models that predict limnetic total P and Chi a from diatom assemblages and nutrient accumulation rates in surface sediments. These models were applied to a 210Pb-dated sediment core from Lake Parker, Polk County, Florida, and a pollen-dated profile from Lake Francis, Highlands County, Florida, to infer historical water quality. Our results show that Lake Parker displayed statistically significant increases in limnetic total P and Chi a concentrations since the 1920s. Lake Francis has been eutrophic for more than 2 millennia, but shows a statistically significant decrease in limnetic total P during the present century. This study demonstrates that lakes may be naturally productive because of edaphic influences and may respond differently to human disturbance. We conclude t...

A multi-proxy trophic state reconstruction for shallow Orange Lake, Florida, USA: Possible influence of macrophytes on limnetic nutrient concentrations

We retrieved four sediment cores from shallow, eutrophic, macrophyte-dominated Orange Lake (A = 51.4 km2, zmax <5 m, zmean < 2 m), north-central Florida, USA. The 210Pb-dated profiles were used to evaluate spatial and temporal patterns of bulk sediment and nutrient accumulation in the limnetic zone and to infer historical changes in lake trophic state. Bulk density, organic matter, total carbon, total nitrogen, total phosphorus and non-apatite inorganic phosphorus (NAIP) concentrations displayed stratigraphic similarities among three of four cores, as did accumulation rates of bulk sediment, organic matter and nutrients. Accumulation rates were slower at the fourth site. Nutrients showed generally increasing rates of accumulation since the turn of the century. Percentages of periphytic diatom taxa increased progressively in the cores after ~ 1930. Diatom-inferred limnetic total P trends were similar among profiles. Eutrophic conditions were inferred for the period prior to the turn of the century. The lake was hypereutrophic in the early decades of the 1900s, but inferred limnetic total P values declined after ~ 1930. Declining inferred limnetic total P trends for the last 60--70 years were accompanied by concomitant increases in accumulation rates of total P and NAIP on the lake bottom. Several lines of evidence suggest that after ~ 1930, phosphorus entering Orange Lake was increasingly utilized by submersed macrophytes. Paleolimnological records from Orange Lake highlight the importance of using multiple sediment variables to infer past trophic state and suggest that aquatic macrophytes can play a role in regulating water-column nutrient concentrations in shallow, warm-temperate lakes.

Cyanobacterial Proliferation is a Recent Response to Eutrophication in Many Florida Lakes: A Paleolimnological Assessment

Abstract Cyanobacteria dominate many highly productive Florida lakes. Algal proliferation often is attributed to eutrophication during the last century, but it is poorly documented because Floridas water-quality monitoring programs became common only after 1980. We interpret paleolimnological data from the sediment cores of 6 productive lakes to determine when cyanobacterial proliferation first occurred, and whether it resulted from natural edaphic influence or from eutrophication caused by human activities. Major algal-pigment groups in sediments were analyzed using pigment-extraction and spectrophotometric techniques. Pigment profiles are compared with WACALIB-derived inferences for limnetic total-P, limnetic chlorophyll a, and trophic-state index values based on sedimented diatoms, and with stable isotope (δ13C & δ15N) signatures of organic matter. Cyanobacterial and algal proliferation increased during recent decades in 5 of the 6 study lakes in response to eutrophication. Two lakes demonstrated some evidence of recovery following nutrient-mitigation programs that reduced sewage and other point-source inputs. Five lakes showed intermittent to moderate cyanobacteria presence in the bottom portion of their cores because of edaphic nutrient supply or early watershed disturbance. One highly productive lake showed no evidence of eutrophication and demonstrated that dense cyanobacterial populations can occur naturally. Relationships were particularly strong among sedimented pigment profiles and diatom-inferred limnetic water-quality profiles. Although cyanobacteria have long-standing presence in some naturally productive Florida lakes, our studies suggest that algal proliferation in many lakes is both recent and abrupt in response to eutrophication. Paleolimnological methods are informative about the timing and causes of cyanobacterial appearance in regions where long-term water-quality data are lacking.

Recent Eutrophication in the Southern Basin of Lake Petén Itzá, Guatemala: Human Impact on a Large Tropical Lake

A 210Pb-dated sediment core from a small bay in the southern basin of Lake Petén Itzá, Guatemala documents recent cultural eutrophication. Increased sediment accumulation beginning ∼1930 A.D. coincided with catchment population growth and was a consequence of watershed deforestation and increased surface run-off. At the same time, geochemical records from the Lake Petén Itzá sediment core indicate increased phosphorus loading and organic matter accumulation. High nutrient concentrations after 1965 A.D. coincided with lower sediment C/N ratios, suggesting an increase in the relative contribution of phytoplankton to the organic matter pool. This inference is confirmed by the dominance of eutrophic and hypereutrophic diatom species. Organic matter δ13C values decreased after 1965 A.D., seemingly contradicting other indicators of recent eutrophication in the southern basin of Lake Petén Itzá. Relatively depleted δ13C values in recent sediments, however, may reflect a contribution from 13C-depleted sewage effluent. Increased δ15N of organic matter after 1965 A.D. indicates changes in the dissolved inorganic nitrogen delivered to the lake. The relatively small increase in δ15N (∼0.6‰ ) is less than might be expected with nitrate loading from sewage and soils, and might be offset by the presence of nitrogen-fixing cyanobacteria with low δ15N values.

Paleolimnological evaluation of historical trophic state conditions in hypereutrophic Lake Thonotosassa, Florida, USA

We used paleolimnological methods to evaluate historical water quality in Lake Thonotosassa, Hillsborough County, Florida, USA. Sediment mapping shows that organic deposits are unevenly distributed in the lake. Two short (<130 cm) sediment cores from the depositional zone were analyzed for radioisotopes (210Pb, 226Ra, and 137Cs), bulk density, organic matter concentration, nutrients (C,N,P), and diatoms. 210Pb results indicate that the profiles represent > 100 years of sediment accumulation. There is an abrupt change in sediment composition at about the turn of the century (∼80 cm depth), above which bulk density decreases and concentrations of organic matter, total C, total N, total P, and 226Ra activity increase. Diatom-based reconstructions of historical water-column trophic conditions indicate progressive nutrient enrichment in the lake during the past ∼100 years. Stratigraphic changes in diatom assemblages suggest that anthropogenic nutrient loading converted Lake Thonotosassa from a naturally eutrophic system to a hypereutrophic waterbody after ∼1900. Given the edaphic setting of Lake Thonotosassa, efforts to mitigate recent anthropogenic impacts will, at best, yield the eutrophic conditions that characterized the lake prior to human disturbance. This study illustrates the importance of paleolimnological data for targeting realistic water quality conditions when lake restoration is contemplated.

Historical Ecology of a Hypereutrophic Florida Lake

ABSTRACT Lake Hollingsworth lies in phosphate-rich central Florida. Recent limnological data show the lake has been hypereutrophic for >25 years, but nothing was known of the lakes trophic history prior to the late 1960s. A paleolimnological study was undertaken to: 1) evaluate the distribution of organic sediments in the basin, 2) reconstruct historical water quality to determine whether the lake is naturally (edaphically) or anthropogenically nutrient-rich, and 3) assess the feasibility of reducing in-lake nutrient concentrations. Organic sediments are distributed rather uniformly above the sandy substrate. Two 210Pb-dated sediment cores taken ≈0.7 km apart display similar stratigraphies with respect to density, organic matter, total C, N, P, radioisotopes, biogenic SiO2, diatoms and δ13C in organic matter. Limnetic Ptot reconstructions, inferred from diatoms, show eutrophic (≈30 μg L−1) conditions in the late 1800s. Total P increased after 1920 and very high Ptot concentrations (125 to >200 μg L−1) ar...