Long-Term Cultural Eutrophication in White and Walden Ponds (Concord, Massachusetts, USA), Thoreau's Lakes of Light

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Long-term cultural eutrophication in White and Walden Ponds (Concord, Massachusetts, USA), Thoreau's lakes of light

J. Curt Stager , Lydia Harvey & Scott Chimileski

To cite this article: J. Curt Stager , Lydia Harvey & Scott Chimileski (2020): Long-term cultural eutrophication in White and Walden Ponds (Concord, Massachusetts, USA), Thoreau's lakes of light, Lake and Reservoir Management, DOI: 10.1080/10402381.2020.1839606 To link to this article: https://doi.org/10.1080/10402381.2020.1839606

Published online: 08 Dec 2020.

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Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=ulrm20 LAKE AND RESERVOIR MANAGEMENT https://doi.org/10.1080/10402381.2020.1839606

Long-term cultural eutrophication in White and Walden Ponds (Concord, Massachusetts, USA), Thoreau’s lakes of light

J. Curt Stager, Lydia Harvey and Scott Chimileski Natural Sciences Division, Paul Smith’s College, Paul Smiths, NY 12970, USA

ABSTRACT KEYWORDS Stager JC, Harvey L, Chimileski S. 2020. Long-term cultural eutrophication in White and Walden Diatoms; eutrophication; Ponds (Concord, Massachusetts, USA), Thoreau’s lakes of light. Lake Reserv Manage. XX:XXX–XXX. paleolimnology; reclamation; Thoreau; Walden Two historically important ponds in the vicinity of Boston, MA, were subjected to a comparative paleolimnological investigation of the timing and causes of eutrophication trends in each. The remarkable clarity of White Pond during the early 19th century led Henry David Thoreau to com- pare it favorably to nearby Walden Pond, but during the 20th century water quality in both ponds declined. Sediment core studies show that cultural eutrophication began at Walden during the 1930s, but no long-term sediment records have been available for White Pond, which makes it more difficult to determine the history and causes of eutrophication there. Here we use microfossil and geochemical analyses of sediment cores to show that major changes in the diatom community of White Pond began around 1900, when fish stocking commenced and soil erosion due to land use in the watershed increased, and that the trend intensified around 1960 and 1990. We also describe efforts to mitigate eutrophication at White Pond, highlight the ecological importance of benthic vegetation in nutrient cycling, and suggest that threats to water quality in both of these ponds will likely increase due to anticipated climatic changes in the region.

Walden Pond (Concord, MA) is a heavily used Walden Pond have been largely successful thus recreational destination for residents of the far (Maynard 2004, Stager et al. 2018). In con- greater Boston area and an international symbol trast, water quality has continued to deteriorate of wild nature due to the writings of Henry at White Pond, where the watershed’s mosaic of David Thoreau (1852, 1854). Less widely known private residences, properties owned by the Town is White Pond, roughly half the size of Walden of Concord, and natural resources managed by and located 4 km to the southwest. Both are deep the state make the situation more complicated. glacial kettle ponds long recognized for their Unlike Walden, White Pond has been closed to exceptional clarity, which led Thoreau (1854)to swimming several times since 2015 due to toxic write that "White Pond and Walden are great cyanobacterial blooms (WPAC 2015,Gerzon2019). crystals, lakes of light." However, like many oligo- Although field studies of limnological condi- trophic waters nationwide (Stoddard et al. 2016), tions in White Pond have been conducted since both ponds have experienced water quality the 1980s (Walker and Ploetz 1988, 1989, 1990, declines since the 19th century (Koster€ et al. ESS Group 2014, 2016–2017, Walker 2017), no 2005, ESS Group 2014, 2016–2017, Walker 2017, longer term environmental records are yet avail- Stager et al. 2018, Gerzon 2019). able to inform its management, unlike the situ- Recent efforts by the Massachusetts ation for Walden Pond, for which sediment core Department of Conservation and Recreation records of limnological conditions cover the last (DCR) to prevent further eutrophication in 1500 years (Winkler 1993,Koster€ et al. 2005,

CONTACT J. Curt Stager [email protected] ß 2020 The Author(s). Published with license by Taylor and Francis Group, LLC This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by- nc-nd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, or built upon in any way. 2 J. C. STAGER ET AL.

Figure 1. Site maps. (Top left) New England with Concord–Boston area highlighted, modified from Wikimedia Commons. (Top right) White and Walden Ponds in relation to Concord and Boston, modified from d-maps.com. (Bottom) Map of the White Pond watershed, with a contour map of the pond and land use classification of surrounding properties. Coring sites within the pond are indicated with stars. Land use map modified from ESS Group (2016–2017), based on Town of Concord parcel data and USGS aerial imagery; contour map modified from MassWildlife (2020). LAKE AND RESERVOIR MANAGEMENT 3

very low concentrations of dissolved oxygen in summer (ESS Group 2014, Walker 2017). Surface pH has ranged between 6 and 7 since the 1970s (Walker and Ploetz, 1988, 1989, 1990), and Secchi disk transparency generally has ranged between 4 and 8 m since 1987 (Walker 2017). Aquatic mosses resembling Fontinalis or Drepanocladus (Figure 2) were retrieved in the tops of sediment cores collected from 10 to 15 m depth at multiple locations in 2017 and 2018, indicating light penetration sufficient to permit Figure 2. Benthic aquatic moss recovered from 15 m depth in photosynthesis on the pond floor. White Pond. Millimeter scale shown above. All hydrological inputs to White Pond derive from groundwater seepage, precipitation, and nearshore runoff. Outputs are by evaporation and Stager et al. 2018). In this article, we summarize groundwater flow-through with a flushing rate of the current state of knowledge of ecological 0.2 times/year (Walker and Ploetz 1988, 1989, changes in White Pond and extend that record ESS Group 2014). Groundwater pumping, includ- several centuries farther back in time with the aid ing that from a town well southeast of the pond, of diatoms (single-celled algae with glassy shells may affect water levels slightly, but surface levels or "frustules") and organic matter preserved in 2 are most strongly correlated with precipitation sediment cores. Microscopic examination of dia- (Walker and Ploetz 1988, 1989, ESS Group 2014, tom frustules in such cores allows the identifica- Walker 2017). Between 1996 and 2016, the level tion of species that reflect past ecological of the pond fluctuated by nearly 2 m conditions, including water clarity, and fluctua- (Walker, 2017). tions in sediment organic content often represent More than 100 private lots with approximately variable soil erosion regimes or changes in lake 3 dozen residences, most of which are occupied productivity. Together, these kinds of analyses year-round, occupy two-thirds of the watershed can provide valuable insights into the environ- (Figure 1; ESS Group 2014). Roughly 5 ha on the mental history of a lake, particularly for time eastern rim of the watershed are cultivated and periods from which direct historical observations are lacking. We also use our findings to address 20 ha of forested conservation and reservation the following questions: (1) How has the diatom lands in the southern sector are owned by the community of White Pond changed since Town of Concord (ESS Group 2014). Public Thoreau’s time? (2) What are the most likely access for recreational activities is provided by a causes of those changes? (3) What can be done state-run boat ramp beside a formerly private to protect water quality in White and Walden beach on the eastern shore and footpaths to an ’ Ponds in the context of future climatic change? undeveloped beach at Sachem s Cove on the southwestern shore (Figure 1). Study site Historical background White Pond (4225’40" N; 7123’27"W) is a flow- through seepage lake of 16 ha that is rimmed Indigenous peoples lived in the region for thou- by steep slopes within a watershed of 46 ha sands of years before Anglo-Europeans estab- (ESS Group 2014). Local bedrock is granitic and lished the Town of Concord in 1635, and White overlain by sandy glacial deposits (Walker and Pond itself was purchased from native residents Ploetz 1989). The pond has a volume of 1.3 mil- in 1684 (Gutteridge 1921, Marquet 1973, Gerzon lion m3, mean depth of 8 m, and maximum depth 2019). However, the time frame most relevant to of 15 m (Figure 1) in 3 subbasins that develop this study began during the early 19th century. 4 J. C. STAGER ET AL.

Thoreau visited White Pond frequently during Massachusetts Department of Fish and Game the early 1800s and produced a detailed bathy- (MADFG; Kaye D, Concord Department of metric map of it, in addition to one of Walden Natural Resources, 2020, pers. comm.), and it Pond (Thoreau 1854, Deevey 1942). At that time continues today. An early record of fish commu- the pond’s beauty and clarity were already nity composition from 1911 (ESS Group 2014) legendary. In Walden; or Life in the Woods listed golden shiners (Notemigonus crysoleucas) (1854), Thoreau wrote that "Since the woodcut- and pumpkinseeds (Lepomis gibbosus), but previ- ters, and the railroad, and I myself have profaned ous stocking had also introduced brown trout Walden, perhaps the most attractive … of all (Salmo trutta) and "pike perch" (possibly Sander our lakes, the gem of the woods, is White Pond." sp.) by then. A seemingly random array of spe- By the 1830s, however, roads bordering the cies was initially stocked for several decades, northeastern corner of the watershed had encour- including yellow perch (Perca flavescens), small- aged more commerce and settlement near the mouth bass (Micropterus dolomieu), largemouth pond (ESS Group 2014). Around 1875, the bass (Micropterus salmoides), bullheads Framingham and Lowell Railroad opened a track (Ameiurus nebulosis), chain pickerel (Esox niger), near the western shore that continued to operate crappie (Pomoxis nigromaculatus), brook trout until the 1970s (Marquet 1973, Gerzon 2019). (Salvelinus fontinalis), bluegills (Lepomis macro- During the early 20th century (and probably chirus), and pumpkinseeds. In 1949, only perch, earlier), farming occurred on what are now resi- smallmouth bass, and bluegills were considered dential lots along Powder Mill Road on the well established, and in 1953 yellow perch were northern edge of the White Pond watershed abundant and also stunted in size, presumably (Figure 1; Gerzon 2019). By the 1920s the pond due to competition for food. had become a "very lively place" due to heavy The pond was poisoned ("reclaimed") with recreational use of its beaches, and in 1930 the rotenone in September 1953, prior to restocking White Pond Association (WPA) was formed, in with rainbow trout (Oncorhynchus mykiss), part, to help protect the shoreline from overuse brown trout, and brook trout. Golden shiners, (Marquet 1973, Gerzon 2019). In 1931, former bullheads, largemouth bass, and pumpkinseeds farmland in the northern and southeastern sec- were reported again in 1956, and the pond was tors of the watershed was subdivided as the Pine reclaimed a second time in September 1958. By Knoll Shores development and sold for summer 2013–2014, bluegills, pumpkinseeds, brown and homes (ESS Group 2014, Gerzon 2019). The rainbow trout, golden shiners, and largemouth WPA maintained a beach on the eastern shore bass were reported yet again (ESS Group 2014). for several hundred members from that time In 1972, an unpublished fisheries report until 2019, when the Town of Concord acquired (MADFG, 1972, unpubl. data) concluded that the property (Kaye D, Concord Department of White Pond was still "clean" but that woodlands Natural Resources, 2020, pers. comm.). Road near the southern shoreline should be protected access to the eastern side of the pond was from development. In response, the Town of improved around 1930 (ESS Group 2014) but Concord established 4 ha of forested White Pond anglers complained of lack of public right-of-way Conservation Land in 1973 and 16 ha of White during the 1930s and 1940s (Kaye D, Concord Pond Reservation Land in 1992 (ESS Group Department of Natural Resources, 2020, pers. 2014). Swimming is discouraged on the town- comm.). From this, it appears that the boat ramp owned shore of Sachem’s Cove, but the rule has (Figure 1) may not have been available to the not been consistently enforced. In 1975, a White public until after 1949. By the 1960s, the water- Pond Advisory Committee was formed to address shed contained a "very high density of residences" pond-related issues, and Friends of White Pond and many of the former summer homes were was established in 1987 in response to continued occupied year-round (ESS Group 2014). concerns over the environmental condition of the Fish stocking commenced at White Pond in pond (Gerzon 2019). In 2019, the town acquired 1903 according to unpublished reports by the the WPA beach, installed the first public sanitary LAKE AND RESERVOIR MANAGEMENT 5

Table 1. Radiocarbon ages of sediment samples from White Pond cores WP-2 and WP-5. Age cali- brations were conducted with CALIB 7.1 (Stuiver and Reimer 1993). Calibrated ages in parentheses are assumed to be anomalously old due to the Suess effect (see text). Core depth, cm 14C age, years B.P. Calibrated years B.P., 2 sigma range ID number 25.25 (WP-2) 505 ± 15 (513-539) OS-135048 32.25 (WP-2) 555 ± 25 530-558, 603-628 OS-135049 40.25 (WP-2) 1200 ± 20 1064-1179 OS-135050 48.25 (WP-2) 1660 ± 25 1525-1618,1679-1684 OS-133689 25.5 (WP-5) 635 ± 20 (557-604, 627-661) OS-136630 facilities there, and constructed access stairways, Materials and methods fencing, and shore stabilization structures at Two sediment cores were collected for analysis ’ Sachem s Cove (Kaye D, Concord Department of from 2 locations within White Pond. Core WP-2 Natural Resources, 2020, pers. comm.). of 50 cm length was collected from 15 m water Frequent limnological monitoring by Walker depth in the central basin in May 2017 (Figure (2017) since 1987 has shown that transparency in 1), using a UWITEC gravity corer. Gravity core summer as measured by Secchi disk has declined WP-5 of 27 cm length was taken from 12.5 m and that it also became less variable during the depth in the eastern basin in July 2018. Both past decade. Transparency formerly ranged cores were extruded vertically in the field at 1 cm between 3 and 10 m but it is now mostly within increments, bagged, and stored under refriger- – the 4 7 m range. The clarity of the pond as ation prior to analysis. The sediment–water inter- viewed from the surface is not necessarily indica- face in all cases appeared to be intact, and WP-5 tive of its overall productivity, however. As early also contained horizontal laminations within the as the 1980s, a "bulge" of oxygen enrichment and uppermost centimeter. high chlorophyll a concentrations indicated high Radiocarbon ages were determined for bulk phytoplankton productivity near the thermocline sediment samples from the cores through acceler- – – around 5 9 m depth (ESS Group 2016 2017, ator mass spectrometry and were converted to Walker 2017). The apparent absence of such a calibrated ages with CALIB 7.1 (Table 1; Stuiver bulge in 1975 (Walker and Ploetz 1988) suggests and Reimer 1993). Age–depth models represent- that mid-depth productivity might have increased ing the last 1–2 centuries (Figure 3) were con- shortly thereafter. structed by measuring 210Pb activity with depth The earliest report of cyanobacterial blooms in and applying the constant rate of supply model White Pond was in summer 1986, when (Appleby and Oldfield 1978). Because the 210Pb Anabaena, Anacystis, and Chroococcus were pre- profile of the shorter WP-5 core suggested that dominant genera (Walker and Ploetz 1988). background activity levels might not have been Summer blooms were also noted in 1987 and reached at the base of the core (Figure 3), the 1988 (ESS Group 2014), and no-swim advisories age-depth model of WP-2 was considered more were issued due to cyanobacterial toxicity in reliable and only WP-2 was subjected to more 2015, 2016, 2017, and 2019 (ESS Group detailed analyses. Organic contents of the sedi- 2016–2017, Gerzon 2019). ments in core WP-2 were estimated by percent It is not known how long deepwater anoxia weight loss on ignition (%LOI) at 500 C has occurred in White Pond, but dissolved oxy- (Sutherland 1998, Heiri et al. 2001). gen concentrations of <1 ppm were measured in Samples for diatom analysis were diluted in the lowest 3 m of the pond during summer 1974 distilled water, dried on coverslips, and mounted (MADFG, 1974, unpubl. data). Extreme oxygen on glass slides with Permount mounting medium. depletion below 15 m depth was also documented Microscopic examination showed no significant in 1988 (Walker and Ploetz 1989), and the oxy- clumping or viewing interference from organic gen-depleted zone (<2 ppm) on the pond bottom films or debris that would require removal by increased in thickness from 2 m to 5 m between chemical treatments, so the subsamples were not 2006 and 2016 (Walker 2017). subjected to further processing that could damage 6 J. C. STAGER ET AL.

Figure 3. Lead-210 and sediment accumulation profiles with age–depth models for White Pond cores WP-2 (left) and WP-5 (right). Dotted lines indicate temporal uncertainty ranges for the age–depth models. microfossils, an approach that has also been used successfully elsewhere (Stager et al. 2018, 2019). A mean of 440 diatom valves per sample from the WP-2 core were identified at 1000 under oil immersion using standard references (Patrick and Reimer 1966, 1975, Krammer and Lange- Bertalot 1991, Diatoms of North America 2019). Siliceous scales of chrysophyte algae were also enumerated along with the diatoms. Diatoms in core WP-5 were enumerated at coarser temporal and taxonomic resolution for comparative purposes only, with a mean of 300 diatoms counted per sample.

Results Lead-210 activity in core WP-2 decreased down- ward roughly exponentially to 16 cm depth, below which uniform background levels were reached (Figure 3). In core WP-5, however, activity dropped more sharply below 5 cm depth and Figure 4. Composite of linear age–depth models for core 210 continued to decrease to levels at the base of the WP-2 using Pb (solid line) and radiocarbon dating (dotted line). The approximated intersection of the age–depth lines at core that did not definitively represent back- 23–24 cm depth suggests that sediment accumulation rates ground (Engstrom D, Science Museum of MN, accelerated during the late 19th century. LAKE AND RESERVOIR MANAGEMENT 7

Figure 5. Stratigraphy of the most common diatom taxa in White Pond core WP-2. Ratios of chrysophyte scales to diatom valves indicated as "Scales:Diatoms." Organic content of sediments indicated by weight loss on ignition (%LOI). Horizontal dotted lines indicate qualitative assemblage zones A–E, as described in the text. STOCK indicates the onset of fish stocking in 1903. REC marks the time frame in which 2 reclamations occurred (see text). Ages of depth intervals based on 210Pb and radiocarbon dating are shown on right margin.

2018, pers. comm.). The 210Pb age models for common in the upper half of the core, along with cores WP-2 and WP-5 suggested that their basal the Lindavia bodanica sensu lato group (mostly L. sediments were deposited during the mid 18th cf. affinis (Grunow) Nakov, Guillory, Julius, E.C. century and circa AD 1900, respectively (Figure Ther., & Alverson), A. formosa Hassal, and Synedra 3). In contrast, the radiocarbon results (Table 1) nana F. Meister. The diatom assemblages and stra- indicated basal ages closer to 1600 and 600 years, tigraphy in core WP-5 were similar (Figure 6). We respectively (Table 1 and Figure 4). visually grouped diatom assemblages in core WP-2 The organic contents (%LOI) in WP-2 ranged into stratigraphic zones for discussion purposes close to 60% between 50 cm and 20 cm depths, here (Figure 5), based upon first appearances or with a minor decrease between 24 and 22 cm depth major increases in ecologically informative taxa. (Figure 5). Above 20cm, the %LOI values declined to the 40% range between 15 and 4 cm depth, then rose again to nearly 60% at the core top. Zone A (50–31 cm; uncertain age to ca. 1840s) The most common diatom taxa in the lower half of WP-2 were Tabellaria flocculosa Roth (Kutzing)€ Approximately half of the diatoms in this basal zone and members of the genera Eunotia and Pinnularia were benthic taxa belonging to the genera Eunotia (Figure 5). Asterionella ralfsii var. americana Korner€ and Pinnularia (mostly P. biceps W. Gregory), along and members of the Discostella stelligera (Cleve & with lesser abundances of other genera including Grunow) Houk & Klee complex were most Frustulia, Navicula, Cymbella,andStauroneis. Also 8 J. C. STAGER ET AL.

Figure 6. Stratigraphy of the most common diatom taxa in supplemental core WP-5. Ratios of chrysophyte scales to diatom valves indicated as "Scales:Diatoms." Horizontal dotted lines indicate major transitions in diatom assemblage composition of nature and timing similar to those in core WP-2. Ages of depth intervals based on 210Pb and radiocarbon dating are shown on right margin. common were varieties of T. flocculosa, particularly Klee, represented more than half of the diatom the planktonic form IIIP sensu Koppen. assemblage. This zone saw declines in relative abundances of Eunotia, Pinnularia, A. ralfsii, and Zone B (31–18 cm; ca. 1840s to 1900) T. flocculosa, accompanied by increases in S. The first appearance of small numbers of the D. nana and other elongated, lightly silicified taxa stelligera complex at 31 cm depth characterized listed here as "Synedra slender species," including this zone. Chrysophyte algal scales also became one resembling unicellular Fragilaria crotonensis slightly more numerous. Kitton. The earliest appearance of A. formosa occurred here, as well. Ratios of chrysophyte scales to diatom valves were low. Zone C (18–9 cm; ca. 1900 to 1960) Elongated A. ralfsii var. americana became abundant and percentages of T. flocculosa decreased. Zone E (4–0 cm; ca. 1990 to 2017) Discostella, Lindavia, S. nana,andAchnanthidium minutissimum (Kutzing) Czarnecki were present Members of the D. stelligera complex were in small amounts, and ratios of chrysophyte scales numerous, and Eunotia, Pinnularia, and T. floc- (mainly in the genus Mallomonas) to diatom culosa remained uncommon. Widely fluctuating valves were the highest of the record. percentages of A. ralfsii, A. formosa, and S. nana each represented up to 20% of the assemblage, and Lindavia, Synedra slender species, and A. – Zone D (9 4 cm; ca. 1960 to 1990) minutissimum ranged up to 10% of the assem- The D. stelligera complex, including both D. stel- blage. Ratios of chrysophyte scales to diatom ligera and D. pseudostelligera (Hustedt) Houk & valves were nearly as high as in Zone C. LAKE AND RESERVOIR MANAGEMENT 9

Figure 7. Diatom stratigraphy of Walden Pond core WAL-3, after Stager et al. (2018). Ratios of chrysophyte scales to diatom valves indicated as "Scales:Diatoms." Organic content of sediments indicated by weight loss on ignition (%LOI). Ages of depth intervals based on 210Pb and radiocarbon dating are shown on right margin.

Discussion circa 1870, the much older radiocarbon age The 210Pb profiles of the 2 cores indicated that obtained for sediment from the same interval WP-2 represented a longer time period than (Table 1) can reasonably be attributed to the WP-5 (Figure 3). That result was expected Suess effect, by which global contamination with because WP-5 was shorter and was also col- the ancient carbon in fossil fuel emissions since lected from a site closer to shore that was more the 18th century has skewed the apparent ages of directly exposed to runoff from the boat ramp organic matter to older dates (Tans et al. 1979). (Figure 1), a setting in which sediment accumu- A radiocarbon date from 25.5 cm depth in core lation rates were likely to be higher WP-5 (Table 1) was also considered anomalous than offshore. due to the Suess effect. Radiocarbon ages of sediments in the lower In contrast, the 3 oldest radiocarbon ages from portion of core WP-2 were much older than WP-2 appeared to be accurate. They represented expected from the 210Pb-derived ages in the time periods that likely preceded the Suess effect, upper 30 cm (Table 1 and Figures 3 and 4), and and contamination of the sediments with ancient they were also significantly older than in a core carbon from local mineral deposits is unlikely to that was collected from similar depth in Walden cause age discrepancies at White Pond because of Pond (Figure 7). Because the 210Pb-derived date the granitic bedrock and siliceous sands of the for the 25.5 cm interval in the WP-2 core was watershed. We find no evidence of stratigraphic 10 J. C. STAGER ET AL.

Figure 8. Summary of major changes in diatom assemblages and organic content in White Pond vs. time, in comparison to Walden Pond and weather records from Bedford, Massachusetts (National Centers for Environmental Information [NOAA] 2017). Percentages of diatoms in core WP-2 (A–D): (A) Eutrophication indicators A. formosa and Synedra species. (B) Centric Discostella and Lindavia species groups. (C) Asterionella ralfsii. (D) Benthic taxa including Eunotia, Pinnularia,andAchnanthidium. (E) Organic content of sediments indicated by weight loss on ignition (%LOI). (F) Percentages of A. formosa and S. nana indicating eutrophication in Walden Pond core WAL-3. Vertical dotted lines highlight approximate dates of major stratigraphic transitions in core WP-2: (1) Onset of the rise in percentages of A. ralsfii ca. 1900. (2) Onset of the rise in centric taxa and decline in benthic diatoms ca. 1960. (3) Onset of the latest rise in diatoms indica- tive of eutrophication during the early 1990s. LAKE AND RESERVOIR MANAGEMENT 11 disturbance because the ages increased systemat- the lake. A more sustained decline in %LOI ically with depth (Figure 4), and the radiocarbon above the 20 cm level in WP-2 was most likely ages of sediments in WP-2 and WP-5 were gen- due to soil erosion after circa 1890 that resulted erally comparable at equivalent depths (Table 1). from increased foot traffic around Sachem’s Cove In addition, the diatom records of the 2 cores and development of the woodlands surrounding were similar (Figures 5 and 6) despite having the pond. Changes in the diatom community been collected from different sites. accompanied the %LOI decline, including 4-fold We therefore conclude that (1) the WP-2 core increases in percentages of A. ralfsii and ratios of represents 1600 years of lake history, and (2) chrysophyte scales to diatoms (Figure 5 and 8). the steeper slope of the age–depth model for core Similar sediment inputs have also been shown to WP-2 that was based on 210Pb dating reflected a influence diatom assemblage composition and major acceleration of sediment inputs to White productivity elsewhere (Koster€ et al. 2005, Maier Pond since the late 19th century (Figure 4). et al. 2018, 2019) and could account for faster sediment accumulation rates since the late 19th Ecological changes in the 19th century century (Figure 4). Thoreau (1854) claimed that Walden Pond was The earliest noteworthy change in the WP-2 dia- less pristine than White Pond during the early to tom record was the first appearance of the D. mid 19th century, an observation that is consist- stelligera complex that defined the boundary ent with our sediment core records. Organic con- between Zone A and Zone B in the core (Figure tents in Walden sediments began to decrease 5). This taxon is common in the plankton of around 1800 (Figure 7), presumably due to forest oligotrophic to mesotrophic lakes and has been disturbance and recreational activity that associated with land clearance and soil erosion increased soil erosion and inputs of wastes from elsewhere (Beck et al. 2016, Wengrat et al. 2019). humans and livestock (Koster€ et al. 2005, Stager Unfortunately, the age–depth model for the et al. 2018). Railroad construction near Walden – 35 25 cm interval remains unresolved due to Pond’s western shore in 1844 would have further conflicting dates where the radiocarbon and increased erosional inputs, as would subsequent 210 Pb series overlap (Figure 4), and the inferred forest fires set by sparks from locomotives. After radiocarbon age of the transition was several cen- sediment organic contents began to decline, 210 turies older than the age derived from the Pb planktonic Asterionella formosa became more series. If the former age is correct, then the common while benthic taxa declined slightly, change occurred close to the time when Concord indicating moderate cultural eutrophication was first settled by non-Indigenous people. If the (Figure 7). latter is correct, then the change occurred during the early to mid 1800s, around the time when Ecological changes in the 20th century Thoreau (1854) described the pond as being extremely clear ("pellucid") but also green in In 1911, a fisheries assessment report (MADFG, color. The green coloration is consistent with 1911, unpubl. data) described White Pond as moderate nutrient enrichment from forest fires, "perfectly clear, like Walden" with the "clearest land clearance, shoreline disturbance, and/or water I have seen." However, the microfossil other human impacts on the watershed, which is record of core WP-2 shows that the diatom com- also consistent with the WP-2 record. munity was already beginning to change signifi- More definitively dated ecological changes cantly by then. occurred at White Pond during the late 19th cen- Percentages of A. ralfsii in White Pond rose tury as local land use intensified. A brief decline sharply around 1900. This planktonic taxon is of %LOI values in core WP-2 that began during considered to be an indicator of acidification in the 1870s (Figures 5 and 8) probably reflected many oligotrophic lakes (Sivarajah et al. 2017, inorganic inputs from soil erosion during con- Stager et al. 2019), and its brief appearance at struction of the railroad near the western edge of Walden Pond during the mid 20th century 12 J. C. STAGER ET AL.

(Figure 7) led Winkler (1993) to interpret it as a percentages of benthic Eunotia and Pinnularia in sign of mild acidification there. However, circum- WP-2 (Figures 5 and 8) are consistent with shad- neutral pH measurements at White Pond during ing by increasingly dense phytoplankton the last half-century revealed no evidence of sig- populations. nificant acidification (Walker and Ploetz 1988, The change in diatoms circa 1960 occurred 1989, 1990). The increase of A. ralfsii in this case shortly after the reclamations of the lake, which more likely resulted from nutrient enrichment indicates a possible causal link. The ecological due to land use and soil erosion, as has been impacts of nutrients released over short time documented elsewhere (Turkia et al. 1998). periods by reclamations can be amplified by The onset of fish stocking in 1903 might also reduced concentrations of dissolved oxygen due have influenced the diatom community through to bacterial decay of the carcasses (Hupfer and top-down trophic effects, waste inputs from Lewandowski 2008,Nurnberg€ 2009, Stager 2018). hatchery fish, mortality during stocking and Once in place, a resultant cycle of anoxia, angling, and/or increased shoreline degradation internal loading of nutrients, and phytoplankton caused by anglers in pursuit of the stocked fish productivity can become self-sustaining (Ekdahl (Carpenter et al. 1985, Lyons et al. 2016, et al. 2004, 2007). Sienkiewicz and Ga: siorowski 2016). Diatom com- The mass of fish killed during the 1953 rec- munities of oligotrophic lakes in Poland have lamation was said by the MADFG to be 98 kg/ha undergone similar changes after fish stocking (MADFG, 1956, unpubl. data), representing (Sienkiewicz and Ga: siorowski 2016), but the 1570 kg in total. If the water content of yellow composition of the fish community and degree of perch is taken to be 80% and the phosphorus (P) fishing pressure at White Pond prior to stocking content is 0.2% of dry weight (Gonzalez et al. are unknown. It is therefore not possible to rigor- 2006), then the reclamation could potentially ously evaluate possible impacts of stocking apart have rapidly mobilized 0.6 kg of biologically from noting the nature and timing of such activ- active P in the water and bottom sediments. That ities in relation to the shift in the diatom record. amount of P is less than the amount (22 kg) that Scales of chrysophyte algae also became more the pond has been said to be capable of absorb- abundant in the sediments circa 1900. Increases ing annually without increasing productivity (ESS of chrysophyte populations in other temperate Group 2014). In 1958, the second reclamation zone lakes have been variously attributed to cli- reportedly killed half as many fish per hectare mate warming (Paterson et al 2001, Ginn et al. (MADFG, 1961, unpubl. data), potentially repre- 2010), pH changes (Paterson et al. 2001, Dixit et senting half as much P-mobilization as in 1953. al. 2002), and land use (Lott et al. 1994). The However, MADFG fisheries staff typically early onset of the change in White Pond makes removed as many fish carcasses as possible fol- local nutrient enrichment the most likely cause lowing reclamations in other ponds in rather than large-scale external factors, particu- Massachusetts (Richards T, MassWildlife, larly in light of enhanced sediment inputs Division of Fisheries and Wildlife, 2019, pers. (reduced %LOI) and fisheries activities then, as comm.). If the same practice was applied to well as the lack of a contemporaneous rise in White Pond, then the nutrient releases from rec- chrysophyte abundances in Walden Pond until lamation should have been negligible. much later (Figure 7). Although effects of fisheries management prac- More pronounced eutrophication began at tices on the phytoplankton community of White White Pond around 1960, as indicated by the Pond cannot be ruled out, the pond’s primary first appearance of A. formosa and increased per- source of long-term nutrient enrichment is the centages of D. stelligera and S. nana (Figures 5 surrounding watershed. The eastern boat ramp and 8), which are common in Walden Pond lacked public sanitary facilities until recently, and (Figure 7) and other moderately productive lakes erosion of slopes and footpaths at Sachem’s Cove (Turkia et al. 1998, Saros et al. 2005, Stager et al. have long added nutrients to the pond, as has the 2018, Wengrat et al. 2019). Decreasing absence of sanitary facilities there. The largest, LAKE AND RESERVOIR MANAGEMENT 13 most continuous sources of P to White Pond are Conversely, a lack of contemporaneous residential septic systems, lawn and garden fertil- changes at White Pond during the 1930s (Figures izer, and surface runoff and soil erosion from the 5 and 8) when the first major increases of A. for- shoreline, trails, boat ramp, and roads (Walker mosa and S. nana occurred at Walden (Figures 7 and Ploetz 1988, 1989, ESS Group 2014). and 8) supports previous conclusions that the Climatic changes are also likely to have con- shift at Walden was due to local cultural tributed to the shift in the diatom community of eutrophication rather than to a regional driver White Pond circa 1990 (Figures 5 and 8) as tem- such as climate change or acid deposition peratures and precipitation increased in (Maynard 2004,Koster€ et al. 2005, Stager et al. Massachusetts, most notably since the 1980s. We 2018). Shoreline stabilization, closure of a nearby are unable to make definitive inferences linking dump that reduced droppings from seagulls, and specific climatic parameters to short-term regulation of the number of visitors were among changes in the abundances of particular taxa, as the effective strategies employed at Walden to has been done with seasonally resolved lake mon- mitigate eutrophication since the DCR took over itoring studies in Scandinavia (Maier et al. 2018, management of the watershed in 1975 2019). However, we do know that thickening of (Maynard 2004). the epilimnion of White Pond in response to summer warming from 2006 to 2016 widened the Lakes of light in the 21st century zone of temperatures unsuitable for coldwater As lakes nationwide lose their transparency to fish (>20 C) from 5 to 8 m in depth (Walker eutrophication and also to "browning" due to the 2017). Surface warming that increases the stabil- release of dissolved organic carbon from soils by ity and duration of thermal stratification in lakes warmer, wetter climates (Seekel et al. 2015a,b, in this manner often contributes to oxygen deple- Solomon et al. 2015, Williamson et al. 2015, tion and internal loading of P from bottom sedi- – Stager et al. 2019), the clarity that waterbodies ments (ESS Group 2014, 2016 2017) that can such as White and Walden Ponds displayed in affect the composition and abundance of diatom the past has become increasingly rare (Stoddard communities. Increased precipitation in the et al. 2016). Although both ponds remain clear in region (Figure 8) along with warming is also comparison to many New England lakes, they likely to have stimulated more nutrient release rarely if ever display the 9–10 m transparencies from soils in the watershed by microbial decay, observed by Thoreau at Walden (Deevey 1942). and heavier storm events would have washed During the last 2 decades, White Pond’s phyto- more nutrients in from surrounding roads plankton community was more taxonomically and slopes. altered than at any time in recent centuries, and In contrast to White Pond, Walden Pond it has also become much more similar to that of experienced no major changes in its diatom or Walden Pond (Figures 5–7). Planktonic A. for- chrysophyte communities when routine stocking mosa and S. nana, indicating more productive of multiple fish species began in 1905 or when conditions (Koster€ et al. 2005, Sienkiewicz and reclamation occurred in 1968 (Figures 7 and 8). Ga: siorowski 2016), are now common and benthic The lack of responses to those practices at diatoms have declined as the water has become Walden suggests that similar fisheries manage- less transparent. The rising organic content of ment activities were not the primary cause of sediments since the mid 1990s (Figures 5 and 8) changes in the diatom community of White Pond is also consistent with increasing productivity, around 1900 (stocking) and 1960 (reclamations). and laminations at the top of core WP-5 indi- However, the volume of White Pond is much less cated recent hypoxia sufficient to prevent sedi- than that of Walden (1.3 million and 3.2 million ment mixing by benthic organisms. Similar m3, respectively; Colman and Friesz 2001, ESS conditions caused sediment laminations to Group 2014), which could make its ecosystem develop due to cultural eutrophication at more sensitive to such perturbations. Crawford Lake, Ontario, as well (Ekdahl et al. 14 J. C. STAGER ET AL.

2004, 2007), and they can also be exacerbated by et al. 2000). Benthic photosynthesis is also a warmer climatic regimes such as those that have source of dissolved oxygen that helps to mitigate developed in New England in recent decades internal loading of P, and it probably contributes (Figure 8). to bulges in dissolved oxygen concentrations at Although the eutrophication trend at Walden depth in both ponds (Colman and Waldron has more or less leveled off since the 1970s 1998, Colman and Friesz 2001, ESS Group 2014, (Figures 7 and 8; Stager et al. 2018), water quality 2016-2017, Walker 2017). Eutrophication and in White Pond has continued to decline. Deep- associated clouding of the water column can water oxygen depletion is also more severe at inhibit benthic vegetation through shading and White Pond than at Walden (Walker and Ploetz thereby lead to a self-amplifying cycle of increas- 1988, ESS Group 2016–2017). Fortunately, local ing oxygen depletion, internal loading, and residents and the Town of Concord have recently greater uptake of a lake’s nutrient budget by launched aggressive efforts to mitigate the trend, phytoplankton (Wagner et al. 2000, Arnold et al. despite having fewer economic and organizational 2019). At Walden, benthic meadows represent far resources readily available than the state-run more biomass than the phytoplankton, meaning management structure at Walden. Those efforts that a large reservoir of nutrients that is currently include shoreline stabilization, installation of trapped on the bottom could become available to public sanitary facilities, and upgrading of resi- plankton if the macrophytes are lost due to shad- dential waste disposal systems. Nonetheless, the ing (Colman and Waldron 1998, Colman and presence of several dozen residences atop the Friesz 2001). The same appears to be true for porous sandy soils surrounding such a small White Pond, as well. waterbody means that chronic nutrient inputs Like a canary in a coal mine, benthic vegeta- from former and current leach fields, lawns, gar- tion can warn of impending shifts in trophic dens, and roadways are now a permanent feature states. However, only limited information on the of the ecosystem. composition, extent, and health of macrophyte Anticipated warming in New England during communities in the ponds is currently available. this century (USGCRP 2017) is likely to further To our knowledge, no comprehensive ecological amplify the risk of eutrophication in both ponds study of the benthic community of White Pond by increasing the intensity and duration of sum- has yet been conducted, but Thoreau collected mer stratification and internal loading of P macrophytes from up to 12 m depth there (Winder and Sommer 2012). Future warming will (Thoreau 1852), and our core samplers retrieved also tend to draw more visitors to the ponds. aquatic mosses at several offshore locations. At Wastes released by bathers currently represent Walden Pond, Thoreau (1854) mentioned finding only a small fraction of the annual phosphorus a "fibrous green weed," possibly Nitella, on boat budget for White Pond (ESS Group 2014), but anchors during the early 19th century, Deevey much larger numbers of swimmers in Walden (1942) dredged living Fontinalis from 15.7 m Pond represent as much as half of the summer depth in 1939, and Winkler (1993) reported phosphorus budget there (Colman and Friesz declining abundances of Isoetes due to shading 2001). If storm events also become increasingly during the 20th century. Greater scrutiny of the severe (USGCRP 2017), more abundant runoff benthic vegetation in both ponds is there- and groundwater seepage will further increase fore warranted. external nutrient inputs to both ponds (Sinha et al. 2017). Monitoring and management of Summary and conclusions trophic status and water clarity will therefore become even more important as climates con- The diatom community of White Pond has tinue to evolve in future (Jeppesen et al. 2017). changed more significantly since the late 1800s Benthic vegetation can help to maintain water than at any other time in recent centuries. clarity by sequestering nutrients on the bottom Formerly dominated by Tabellaria flocculosa, the beyond the reach of phytoplankton (Wagner planktonic component of the community has LAKE AND RESERVOIR MANAGEMENT 15 become more diverse and characterized by taxa Acknowledgments that likewise reflect cultural eutrophication in Financial and technical support for this project was pro- Walden Pond. The timing of major shifts in dia- vided by the Draper-Lussi Endowment and Paul Smith’s tom assemblages circa 1900 and 1960 indicates College. We thank Delia Kaye, Christine Gerzon, and Todd that fisheries management practices might have Richards for providing background information and litera- contributed somewhat to those changes. ture regarding the histories of White Pond and Walden Pond, and William Walker kindly provided feedback on the However, chronic nutrient inputs from recre- manuscript. Paul Smith’s College students Matt Spadoni ational activities and settlement in the watershed and David Prosser assisted with sediment coring. Lead-210 were probably more important in causing the dating was performed by Dan Engstrom and colleagues, and overall eutrophication trend during the 20th cen- radiocarbon dating was performed by NOSAMS staff at tury. Together, the combined effects of fisheries Woods Hole Oceanographic Institute. management, nutrient enrichment, and climate change have contributed to a stepwise evolution of References phytoplankton assemblages that has pushed White Appleby PG, Oldfield F. 1978. The calculation of lead-210 Pond into successively higher trophic states. dates assuming a constant rate of supply of unsupported Degradation of water quality at Walden Pond 210Pb to the sediment. CATENA 5(1):1–8. has been largely mitigated in recent decades by Arnold TE, Brenner M, Kenney WF, Bianchi TS. 2019. shoreline stabilization, closure of a local landfill, Recent trophic state changes of Florida lakes inferred from bulk sediment geochemical variables and bio- provision of sanitary facilities, and other efforts markers. J Paleolimnol. 62(4):409–423. by the DCR. In contrast, eutrophication has con- Beck KK, Medeiros AS, Finkelstein SA. 2016. Drivers of tinued at White Pond to the point that cyanobac- change in a 7300-year Holocene diatom record from the terial blooms have become a frequent health hemi-boreal region of Ontario, Canada. PLOS One. hazard, and benthic anoxia has become more 11(8):e0159937. doi:10.1371/journal.pone.0159937. Carpenter SR, Kitchell JF, Hodgson JR. 1985. Cascading extreme than at Walden. The town and shoreline trophic interactions and lake productivity. BioScience. residents have recently increased the availability 35(10):634–639. of sanitary facilities for visitors and stabilized Colman JA, Waldron MC. 1998. Walden Pond, some of the slopes and footpaths around White Massachusetts: environmental setting and current investi- Pond, as was done previously at Walden gations. USGS Fact Sheet FS-064-98 (Maynard 2004). Nonetheless, anticipated long- Colman JA, Friesz PJ. 2001. 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