The PIRLA project: Paleoecological investigations of recent lake acidification

The PIRLA project is a broadly interdisciplinary paleolimnological investigation of five to fifteen comparable watershed/lake systems from each of four low-alkalinity regions in North America that are currently receiving acid deposition. The areas are the Adirondack Mountains (N.Y.), northern New England, northern Great Lakes states, and northern Florida. The primary objective of the study is to provide a detailed reconstruction of the recent acidification histories of a representative suite of lakes from each of the regions. The study will increase our understanding of the timing, rates, and magnitude of acidification (and other chemical changes), and the regional and inter-regional patterns of lake acidification.     

Spatial and temporal patterns of sediment and nutrient accumulation in shallow lakes of the Upper St. Johns River Basin,Florida

We used paleolimnological methods to investigate spatial and temporal patterns of bulk sediment and nutrient (C, N, P) accumulation in Lakes Hell ‘n’ Blazes (A = 154 ha, zmax = 240 cm), Sawgrass (A = 195 ha, zmax = 157 cm) and Washington (A = 1766 ha, zmax = 322 cm), in the Upper St. Johns River Basin, Florida. The study was designed to evaluate long-term changes in sedimentation and nutrient storage in the basin, and was one component of a larger project addressing flood control, wetland restoration, and water quality improvement. These three study lakes are wide, shallow waterbodies in the upper reaches of the St. Johns River channel. Sediment mapping indicates soft, organic deposits are distributed uniformly throughout Lakes Hell ‘n’ Blazes and Sawgrass. In contrast, much of Lake Washington is characterized by sandy bottom, and organic sediment is largely restricted to the north end of the lake. Lakes Hell ‘n’ Blazes and Sawgrass are effective sediment traps because dense submersed macrophytes and their associated epiphytes reduce flow velocity, intercept suspended particles, and utilize dissolved nutrients. Abundant Hydrilla, combined with short fetch, prevents resuspension and downstream transport of sediments. Larger Lake Washington is probably wind-mixed and resuspended organic sediments are redeposited to downstream sites. 210Pb-dated sediment cores show that organic sediment accumulation began in all three lakes before 1900, but that bulk sediment and nutrient accumulation rates have generally increased since then. The increases are probably attributable, in part, to anthropogenic activities including 1) hydrologic modifications that reduced flow rates in the channel, 2) discharge of nutrient-rich waters from urban, agricultural and ranching areas, and, 3) introduction and periodic herbicide treatment of the exotic macrophytes Eichhornia and Hydrilla.     

Cesium-137 sediment depth profiles and inventories in Adirondack Lake sediments

Depth distributions and inventories of¹³⁷Cs (mCi km²) were determined in sediment from several fresh water lakes in the New York State Adirondack Preserve. Included were Big Moose and Darts Lakes, part of the North Branch of the Moose River system, as well as North, Sagamore, South, and Woods Lakes and the seepage pond, Tamarack Lake. Comparisons were made between the¹³⁷Cs inventories in these lakes and large inpoundments in the Adirondacks (Hinkley, Great Sacandaga, Stillwater and Cranberry Lake Reservoirs) and other large impoundments and lakes located in various regions of the U.S., especially Cayuga Lake, Ithaca, NY.None of the Adirondack Lakes had¹³⁷Cs distributions with depth in sediment that closely resembled the deposition pattern of weapons testing as a function of time. All of the natural lakes and small impoundments, including the seepage pond, were found to have significantly lower inventories of¹³⁷Cs than expected; while the large reservoirs were generally enhanced in¹³⁷Cs. We suggest that more than one mechanism may be responsible for the low sediment inventories: for the majority of lakes, flushing of¹³⁷Cs out of the lakes during periods of thermal stratification and ice thaw; and for the seepage pond, remobilization of¹³⁷Cs into the water column due to biological recycling.     

Sulfur distribution in lake sediment profiles as an index of historical depositional patterns

Changes in S concentration in sediment profiles from South Lake (Adirondack region, New York, USA) and Ledge Pond (western Maine, USA) indicated an increased input of S beginning around 1850. This S was probably derived from anthropogenic sources. The S budget for South Lake indicated that sedimentation of seston rich in S (0.9% dry mass) accounts for most of the input of S in sediment. Sulfur in both seston and sediment is primarily organic (57 and 22% ester sulfate, 32 and 52% carbon-bonded S, respectively). The transformation and translocation of organic S constitutes a major pathway of S flux in South Lake.     

Sedimentary diatom concentrations and accumulation rates as predictors of lake trophic state

Diatom concentrations in surface sediments are positively correlated with limnetic chlorophyll a concentrations in Florida (USA) lakes. Using this relationship, I examine models that provide quantitative inferences for trophic state in historical applications.The best model predicts chlorophyll a trophic state index (TSI) values from log-transformed diatom concentrations and explains approximately half the variance in the dependent variable. Diatom accumulation rates are not better than sedimentary diatom concentrations as predictors of TSI. The entire diatom assemblage is as sensitive an indicator of TSI as are the planktonic diatoms alone. A model that considers the ecological preferences of specific taxa was found to be a better predictor than the model based on total diatom concentration.The sedimentary diatom concentration model provides a useful method for assessing historical changes in primary productivity, except in lakes where factors (e.g., silica limitation, blue-green bacterial inhibition) limited diatom production, or post-depositional changes removed sedimentary diatoms. TSI inferences are presented for sediment cores from two Florida lakes, one of which demonstrates a problematic application, and the other of which does not.     

Material transfer from water to sediment in Florida lakes

We describe statistical relationships between chemical aspects of surficial sediments from 34 Florida lakes, and trophic state of the overlying waters. Trophic state is expressed by Carlson's TSI-Chl a. The objectives of the study are two-fold: 1) to understand processes that govern the transfer of material across the mudwater interface, and 2) to develop transfer functions for inferring historical TSI measures in chemically analyzed, ²¹⁰Pb-dated cores. Simple regression of organic matter content or nutrient (C, N, P) concentration in surface sediments vs. TSI yields nonsignificant or weak positive correlations. However, using a novel application of ²¹⁰Pb assay, net accumulation rates of the materials are estimated and show a better correlation with trophic state. Cation (Ca, Mg, Fe, K) and sulfur concentrations in surface muds are poorly related to corresponding TSI's. Net accumulation rates of these elements (Ca, Mg, Fe, K, S) are positively correlated (P < 0.01) with TSI-Chl a. Chemical data from the Florida surficial sediment survey suggest that inferred levels of past trophic state should be based on net accumulation rates of chemical constituents rather than their simple sediment concentrations.     

Spatial and temporal patterns of sediment and nutrient accumulation in shallow lakes of the Upper St. Johns River Basin, Florida

We used paleolimnological methods to investigate spatial and temporal patterns of bulk sediment and nutrient (C, N, P) accumulation in Lakes Hell n Blazes (A = 154 ha, zmax = 240 cm), Sawgrass (A = 195 ha, zmax = 157 cm) and Washington (A = 1766 ha, zmax = 322 cm), in the Upper St. Johns River Basin, Florida. The study was designed to evaluate long-term changes in sedimentation and nutrient storage in the basin, and was one component of a larger project addressing flood control, wetland restoration, and water quality improvement. These three study lakes are wide, shallow waterbodies in the upper reaches of the St. Johns River channel. Sediment mapping indicates soft, organic deposits are distributed uniformly throughout Lakes Hell n Blazes and Sawgrass. In contrast, much of Lake Washington is characterized by sandy bottom, and organic sediment is largely restricted to the north end of the lake. Lakes Hell n Blazes and Sawgrass are effective sediment traps because dense submersed macrophytes and their associated epiphytes reduce flow velocity, intercept suspended particles, and utilize dissolved nutrients. Abundant Hydrilla, combined with short fetch, prevents resuspension and downstream transport of sediments. Larger Lake Washington is probably wind-mixed and resuspended organic sediments are redeposited to downstream sites. 210Pb-dated sediment cores show that organic sediment accumulation began in all three lakes before 1900, but that bulk sediment and nutrient accumulation rates have generally increased since then. The increases are probably attributable, in part, to anthropogenic activities including 1) hydrologic modifications that reduced flow rates in the channel, 2) discharge of nutrient-rich waters from urban, agricultural and ranching areas, and, 3) introduction and periodic herbicide treatment of the exotic macrophytes Eichhornia and Hydrilla.     

Historical changes in epilimnetic phosphorus concentrations in six rural lakes in Northern Ireland

1. Sediment cores were taken from six lakes in Northern Ireland: Loughs Heron, Ballywillin, Corbet, Patrick, Brantry and Creeve. The present-day total phosphorus (TP) concentrations of these lakes range from around 30 to 400 μg TP l^–1. None of the lakes have sewage point-sources disposing into them, and all have primarily agricultural catchments. 2. Sediment cores were dated using ²¹⁰Pb. The cores cover time periods ranging from about 100 years at Lough Corbet to periods considerably greater than the range of ²¹⁰Pb (more than 150 years), Lough Patrick. 3. Diatom stratigraphies indicate considerable ecological change at all the lakes, regardless of their current nutrient status. Epilimnetic phosphorus concentrations were inferred using weighted averaging regression and calibration. Diatom-inferred TP values for the core surface samples agreed reasonably well with the contemporary values. Pre-1900 TP concentrations range from about 10 μg TP l^–1 at Lough Patrick to 80 μg TP l^–1 at Lough Corbet. The higher values prior to 1900 are assumed to relate to land-clearance activities and the expansion of agriculture associated with the Plantation period. 4. Although there are increasing diatom-inferred TP concentrations between 1850 and 1900 at some sites, there is a final and more marked increase at all lakes after 1950. Phosphorus concentrations have increased at all lakes, by between a factor of three to five, over an average 50-year period. At four of the lakes, TP concentrations have been increasing linearly from around 1950 to the present day, suggesting that, in general, there is little sign of amelioration of the eutrophication problem. 5. The post-1950 increases are assumed to relate to changes in agricultural activity within their catchments, notably the continued improvement in field drainage, which has facilitated transfer of nutrients to water courses and lakes and increasing soil-P concentration due to a positive annual-P balance. Another factor, probably associated with increased animal grazing densities and improvement of grassland, is the application of animal slurries, which appears to be a largely unquantified factor.     

Paleolimnological reconstruction of the effects of atmospheric deposition of acids and heavy metals on the chemistry and biology of lakes in New England and Norway

Sediment cores from nine lakes in southern Norway (N) and six in northern New England (NE) were dated by ¹³⁷Cs, ²¹⁰Pb and in NE also by pollen, and were analyzed geochemically and for diatoms. Cores from two N and three NE lakes were analyzed for cladocerans. ¹³⁷Cs dating is unreliable in these lakes, probably due to mobility of Cs in the sediment. In Holmvatn sediment, an up-core increase in Fe, starting ca. 1900, correlates with geochemical indications of decreasing mechanical erosion of soils. Diatoms indicate a lake acidification starting in the 1920's. We propose that soil Fe was mobilized and runoff acidified by acidic precipitation and/or by soil acidification resulting from vegetational succession following reduced grazing. Even minor land use changes or disturbances in lake watersheds introduce ambiguity to the sedimentary evidence relating to atmospheric influences. Diatom counts from surface sediments in 36 N and 31 NE lakes were regressed against contemporary water pH to obtain coefficients for computing past pH from subsurface counts. Computed decreases of 0.3–0.8 pH units start between I890 and I930 in N lakes already acidic (pH 5.0–5.5) before the decrease. These and lesser decreases in other lakes start decades to over a century after the first sedimentary indications of atmospheric heavy metal pollution. It is proposed that the acidification of precipitation accompanied the metal pollution. The delays in lake acidification may be due to buffering by the lakes and watersheds. The magnitude of acidification and heavy metal loading of the lakes parallels air pollution gradients. Shift in cladoceran remains are contemporary with acidification, preceding elimination of fishes.     

Contribution of Sediment Respiration to Summer CO2 Emission from Low Productive Boreal and Subarctic Lakes

We measured sediment production of carbon dioxide (CO₂) and methane (CH₄) and the net flux of CO₂ across the surfaces of 15 boreal and subarctic lakes of different humic contents. Sediment respiration measurements were made in situ under ambient light conditions. The flux of CO₂ between sediment and water varied between an uptake of 53 and an efflux of 182 mg C m⁻² day⁻¹ from the sediments. The mean respiration rate for sediments in contact with the upper mixed layer (SedR) was positively correlated to dissolved organic carbon (DOC) concentration in the water (r² = 0.61). The net flux of CO₂ across the lake surface [net ecosystem exchange (NEE)] was also closely correlated to DOC concentration in the upper mixed layer (r² = 0.73). The respiration in the water column was generally 10-fold higher per unit lake area compared to sediment respiration. Lakes with DOC concentrations <5.6 mg L⁻¹ had net consumption of CO₂ in the sediments, which we ascribe to benthic primary production. Only lakes with very low DOC concentrations were net autotrophic (<2.6 mg L⁻¹) due to the dominance of dissolved allochthonous organic carbon in the water as an energy source for aquatic organisms. In addition to previous findings of allochthonous organic matter as an important driver of heterotrophic metabolism in the water column of lakes, this study suggests that sediment metabolism is also highly dependent on allochthonous carbon sources.     

Deepwater sediments and trophic conditions in Florida lakes

Sediment cores were taken from near maximum depth in 15 Florida lakes representing a wide range of trophic conditions. Chemical analyses of surface sediments showed Al, Fe, and Ca to be the most abundant elements in all samples, and the ratio of Al to Ca to be smaller for eutrophic lakes. Sediment organic matter increased with trophic state, as did the degree to which it was enriched in nitrogen. Corresponding sediment C/N ratios decreased with increasing lake trophic state and showed significant negative correlation with chlorophylla, total N, and total P in the water column. Concentrations of sedimentary chlorophyll derivatives showed some relation to trophic state but differences in basin morphometry hinder its use as an inter-lake index of chlorophyll production.     

Effects of light and nutrients on the net accumulation and elemental composition of epilithon in boreal lakes

1. Two experiments in the Experimental Lakes Area (ELA) in north-western Ontario, Canada examined the effects of light and two key elements on the net accumulation and elemental composition of epilithon. In Lake (L) 224, benthic algae were grown under different light intensity and phosphorus supply, while in L302S we provided three levels of two different carbon sources (bicarbonate and glucose) to algae colonizing nutrient-diffusing substrata. After 1 month of accumulation, we sampled biofilms for chlorophyll (chl), carbon (C), phosphorus (P) and algal C.
2. Increased C supply did not significantly affect algal growth (C or chl) or elemental composition (C/P ratios) in L302S. However, P enrichment increased chl and algal C, dramatically reduced the C/P ratio of epilithon, and did not affect total organic C in L224. Phosphorus enrichment also increased the proportion of algal material in the total particulate organic matter and altered the taxonomic composition of algae in L224 biofilms. Shading had no significant effect on the C/P ratio and total organic C in epilithon from the L224 experiment.
3. Our results demonstrate that P supply affects the elemental composition of organic matter that collects on rock substrata. It thus appears that low availability of P relative to C and light drives the formation and retention of high C/P organic matter on rock surfaces in oligotrophic boreal lakes.     

Sediment chemistry of lakes formed by surface-mining for coal in the midwestern U.S.A.

Sediment from lakes on abandoned coal mines in the Midwestern U.S.A. was examined to determine the factors controlling chemical composition and the role the sediment plays in lake neutralization. Sediment concentrations of many cations, (especially heavy metals) are strongly correlated with sediment sulfide concentration, but poorly correlated with the pH of the overlying water. Leaching the sediment of one lake with 1 N ammonium acetate, 0.1 N HCl, and 6 N HCl revealed that cations were mostly bound in weak acid-leachable and strong acid-leachable forms. The weak acid-leachable form is likely to be metal sulfides and calcium carbonate. The sulfide-poor sediments of extremely acid lakes contained few weak acid-leachable cations. Raw mine-spoil contained large amounts of easily leached cations. There is little relationship between changes in sediment chemistry over time determined from cores of lake sediment and past lake pH. Rates of sulfide deposition were examined in sediment cores because sulfate reduction and deposition has been suggested as a major source of alkalinity in lakes influenced by acid precipitation. Although the rate of sulfate deposition in surface mine lakes is high, it alone seems to be insufficient to cause neutralization.     

Tracking Recovery Patterns in Acidified Lakes: A Paleolimnological Perspective

Long-term data are often lacking to effectively assess patterns of lake acidification and recovery. Fortunately, paleolimnological techniques can be used to infer past changes in lakewater acidity and related variables by means of biological indicators, such as diatom valves and chrysophyte scales, preserved in ²¹⁰Pb-dated sediment cores. We summarize paleolimnological data that we have gathered from 36 Sudbury (Ontario) and 20 Adirondack Park (New York) lakes to estimate the magnitude of lake acidification and any subsequent recovery in these lake systems. In both regions, many lakes were shown to have acidified considerably, some over two pH units, since the 1850s. Although some recovery was noted in both lake regions, Sudbury lakes generally showed larger increases in inferred lakewater pH with recent declines in sulfur emissions. Possible explanations of these differences include the greater decrease in sulfate deposition in the Sudbury area, as well as generally longer residence times of lakes in Sudbury, perhaps allowing for more in-lake alkalinity generation. In addition, Sudbury lakes generally had higher pre-industrial pH levels, suggesting that lakes with higher natural buffering capacities are more likely to recover more quickly with declines in deposition, even if they had been acidified to a great extent.     

High predation is of key importance for dominance of small-bodied zooplankton in warm shallow lakes: evidence from lakes, fish exclosures and surface sediments

The mean body size of limnetic cladocerans decreases from cold temperate to tropical regions, in both the northern and the southern hemisphere. This size shift has been attributed to both direct (e.g. physiological) or indirect (especially increased predation) impacts. To provide further information on the role of predation, we compiled results from several studies of subtropical Uruguayan lakes using three different approaches: (i) field observations from two lakes with contrasting fish abundance, Lakes Rivera and Rodó, (ii) fish exclusion experiments conducted in in-lake mesocosms in three lakes, and (iii) analyses of the Daphnia egg bank in the surface sediment of eighteen lakes. When fish predation pressure was low due to fish kills in Lake Rivera, large-bodied Daphnia appeared. In contrast, small-sized cladocerans were abundant in Lake Rodó, which exhibited a typical high abundance of fish. Likewise, relatively large cladocerans (e.g. Daphnia and Simocephalus) appeared in fishless mesocosms after only 2 weeks, most likely hatched from resting egg banks stored in the surface sediment, but their abundance declined again after fish stocking. Moreover, field studies showed that 9 out of 18 Uruguayan shallow lakes had resting eggs of Daphnia in their surface sediment despite that this genus was only recorded in three of the lakes in summer water samples, indicating that Daphnia might be able to build up populations at low risk of predation. Our results show that medium and large-sized zooplankton can occur in subtropical lakes when fish predation is removed. The evidence provided here collectively confirms the hypothesis that predation, rather than high-temperature induced physiological constraints, is the key factor determining the dominance of small-sized zooplankton in warm lakes.     

Ecological correlates of net accumulation rates of Cladocera remains in lake sediments

Paleolimnological study of plant and animal microfossils is based on either relative abundances or net accumulation rates of various taxa. Interpretation of the latter is difficult because ecological factors that influence accumulation rates have been unknown. I use a novel method of ²¹⁰Pb assay to estimate net accumulation rates of bosminids and chydorids at the sediment-water interface of 15 Florida (U.S.A.) lakes which represent a wide range of trophic state, water chemistry, and morphometry. Thirty-two species of Chydoridae, and two genera of Bosminidae were recovered from the lake sediments. Accumulation rates of 6 of the 11 more abundant chydorid species and both bosminid taxa were positively correlated with trophic state as measured by chlorophyll a concentration. Lake size was positively correlated with accumulation rates of 4 chydorid species and both bosminids. Weedbed area was positively correlated with accumulation rates of 2 chydorids, but neither bosminid. Lakewater pH was positively correlated with only one chydorid species accumulation rate. Multiple regression models explain statistically nearly all of the variance in each of the limnological variables, and can be used as transfer functions for paleolimnological inference.     

Seasonal phosphorus dynamics in the surficial sediment of two shallow temperate lakes: a solid-phase and pore-water study

This study examined phosphorus (P) dynamics by surveying the solid-phase and pore-water of the surficial sediment in a mesotrophic and eutrophic shallow lake in Maine, USA. Both lakes were dimictic, developed hypolimnetic anoxia, and released sedimentary P. We examined the controls on sedimentary P release by considering two possible mechanisms; mineralization and release of sedimentary organic P, and dissolution of Fe hydroxide following the onset of anoxia. The temporal investigation of solid-phase included sequential chemical extraction and ³¹P NMR analysis, and the pore-water included use of equilibrium samplers. In both lakes, the relative contribution of organic P to total sedimentary P release was minor compared to Fe hydroxide-associated P. The eutrophic lake, however, had more evidence of microbial uptake of sedimentary P and a higher degree of rapid P transformation in the water column. Sediment polyphosphates were dynamic and possibly contributed to hypolimnetic P accumulation. The pore-water Fe and P profiles exhibited similar temporal patterns as the solid-phase results. Together, they showed an upward migration of the redoxcline in the sediment, from winter to summer, resulting in the accumulation of Fe-bound P at the sediment?Cwater interface in the winter followed by its release into the summer.     

Iron:phosphorus ratio in surface sediment as an indicator of phosphate release from aerobic sediments in shallow lakes

Analysis of Danish lakes showed that both mean winter and mean summer concentrations of lake water total phosphorus in the trophogenic zone correlated negatively with the total iron to total phosphorus ratio (Fe:P) in surface sediments. No correlation was found between the water total phosphorus concentration and either the sediment phosphorus concentration alone or with sediment calcium concentration. The increase in total phosphorus from winter to summer, which is partly a function of net internal P-loading, was lowest in lakes with high Fe:P ratios in the surface sediment.A study of aerobic sediments from fifteen lakes, selected as representative of Danish lakes with respect to the sediment Fe and phosphorus content, showed that the release of soluble reactive phosphorus was negatively correlated with the surface sediment Fe:P ratio. Analysis of phosphate adsorption properties of surface sediment from 12 lakes revealed that the capability of aerobic sediments to buffer phosphate concentration correlated with the Fe:P ratio while the maximum adsorption capacity correlated with total iron. Thus, the Fe:P ratio may provide a measure of free sorption sites for orthophosphate ions on iron hydroxyoxide surfaces.The results indicate that provided the Fe:P ratio is above 15 (by weight) it may be possible to control internal P-loading by keeping the surface sediment oxidized. Since the Fe:P ratio is easy to measure, it may be a useful tool in the management of shallow lakes.     

Sediment type and the clonal size greatly affect the asexual reproduction,productivity, and nutrient absorption of Vallisneria natans

Most aquatic vegetation restorations involve the transplantation of submerged macrophytes. Sediment type and the clonal size are of great significance as they determine the fate of submerged macrophytes. In order to ensure successful restoration, a simulation experiment was conducted using aquarium mesocosms to investigate the response of stolon propagation capacity, the morphological features and productivity of Vallisneria natans for four types of sediment (lake mud [L], lake mud + sand [L + S, 50:50, v/v mixture], sand [S], clay [C]), and three types of clonal sizes. Results showed that sediment types significantly affected V. natans biomass accumulation, stolon propagation ability, ramet morphological characteristics, and productivity, where the asexual reproduction ability and productivity ranked as L > L + S > S > C in four sediment types. Total biomass, maximum net production, number of ramets, root diameter, number of stolons, and stolon propagation rate were all highest in L. In L and L + S, the plant chlorophyll content was higher than in S and C. The root diameter and the ratio of aboveground/underground biomass in S were the smallest among the four sediments. Moreover, when more V. natans seedlings were linked, more ramets and biomass were produced. The stolon propagation rate was ranked as the stolon with single seedling greater than the stolon with two‐linked seedlings greater than the stolon with three‐linked seedlings in L and L + S. The concentration of total nitrogen, total phosphorus, and NO₃^?‐N in water was remarkably reduced in four aquariums. Findings provide a scientific basis for restoring submerged macrophytes in different sediment settings.     

Classification of threespine stickleback along the benthic-limnetic axis

Many species of fish display morphological divergence between individuals feeding on macroinvertebrates associated with littoral habitats (benthic morphotypes) and individuals feeding on zooplankton in the limnetic zone (limnetic morphotypes). Threespine stickleback (Gasterosteus aculeatus L.) have diverged along the benthic-limnetic axis into allopatric morphotypes in thousands of populations and into sympatric species pairs in several lakes. However, only a few well known populations have been studied because identifying additional populations as either benthic or limnetic requires detailed dietary or observational studies. Here we develop a Fisher's linear discriminant function based on the skull morphology of known benthic and limnetic stickleback populations from the Cook Inlet Basin of Alaska and test the feasibility of using this function to identify other morphologically divergent populations. Benthic and limnetic morphotypes were separable using this technique and of 45 populations classified, three were identified as morphologically extreme (two benthic and one limnetic), nine as moderately divergent (three benthic and six limnetic) and the remaining 33 populations as morphologically intermediate. Classification scores were found to correlate with eye size, the depth profile of lakes, and the presence of invasive northern pike (Esox lucius). This type of classification function provides a means of integrating the complex morphological differences between morphotypes into a single score that reflects the position of a population along the benthic-limnetic axis and can be used to relate that position to other aspects of stickleback biology.