Evidence from Hawes Water, UK

Freshwater Biology (2015) 60, 2226–2247 doi:10.1111/fwb.12650

Ecological sensitivity of marl lakes to nutrient enrichment: evidence from Hawes Water, UK

† ‡ § EMMA WIIK* , HELEN BENNION*, CARL D. SAYER*, THOMAS A. DAVIDSON , ¶ †† SUZANNE MCGOWAN **, IAN R. PATMORE* AND STEWART J. CLARKE *Environmental Change Research Centre, Department of Geography, University College London, London, U.K. † Institute of Environmental Change and Society, Research and Innovation Centre, University of Regina, Regina, SK Canada ‡ Lake Group, Department of Bioscience, Aarhus University, Silkeborg, Denmark § Section for Ecoinformatics and Biodiversity, Department of Bioscience, Aarhus University, Aarhus, Denmark ¶ School of Geography, University of Nottingham, Nottingham, U.K. **School of Geography, University of Nottingham Malaysia Campus Jalan Broga, Selangor Darul Ehsan, Malaysia †† The National Trust, Bury St Edmunds, Suffolk, U.K.

SUMMARY

1. Highly calcareous (marl) lakes are infrequent but important freshwater ecosystems, protected under the EU Habitats and Species Directive. Chara lakes have been considered resistant to eutrophication owing to the self-stabilising properties of charophyte meadows. However, the opposite is suggested by the large-scale biodiversity declines in marl lake taxa in Europe, and evidence of charophyte sensitivity to eutrophication. We combined contemporary, palaeolimnological and archival methods to investigate the eutrophication of Hawes Water, a shallow marl lake in north-west England (U.K.). 2. Changes in aquatic macrophyte and invertebrate communities were reconstructed through the analysis of historical macrophyte surveys and sedimentary plant and animal macrofossils in two dated sediment cores from the littoral and deep zones of the lake. In addition, chlorophyll and carotenoid pigments were analysed to track changes in primary production from benthic and pelagic areas. Substantial changes in macrophyte communities were detected over centennial timescales, suggesting high ecosystem sensitivity considering the presently moderate phosphorus concentrations in À Hawes Water (mean annual total phosphorus 20 lgL 1). 3. Two apparent periods of threshold-like change were identified from the sediment record: (i) changes in cyanobacteria (aphanizophyll + myxoxanthophyll to canthaxanthin + zeaxanthin) and potentially in nutrient stoichiometry, reductions in the maximum macrophyte colonisation depth and water clarity, reduced charophyte and Potamogeton diversity, and increases in Nymphaeaceae; and (ii) severe reductions in light availability inferred from subdecadal doubling in phytoplankton abundance, substantial increases in Daphnia abundance and the extinction of charophytes from higher water depths. 4. Further, change in both the littoral and deeper water has confined key marl lake taxa to smaller niches. In the littoral, increasing siltation and reed and Nymphaeaceae densities caused extinction of Littorella uniflora in the early 1900s and have reduced the evenness of Characeae with suspected imminent extinction of two highly localised Chara spp. In the deeper water, upslope creep of maximum colonisation depth has reduced habitat for intermediate-depth marl lake taxa leading to the loss of four Potamogeton and one Chara species, and replacement of these taxa by Nuphar lutea. 5. The large changes in macrophyte community composition and increased incidences of turbid water have reduced the distinctive and valued marl lake features of Hawes Water, indicating that marl lakes can, as a habitat type, be highly sensitive to eutrophication. The persistence of abundant generalist macrophyte species at considerable water depth may be a feature of high-alkalinity lakes in clearwater, macrophyte-dominated states, but is a distinct eutrophication response in marl lakes rather than an indication of resistance to eutrophication.

Correspondence: Emma Wiik, Laboratory Building, Department of Biology, University of Regina, 3737 Wascana Parkway, Regina, Saskatche- wan, S4S0A2 Canada. E-mail: [email protected]

2226 © 2015 The Authors Freshwater Biology Published by John Wiley & Sons Ltd. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Evidence for eutrophication in a low-nutrient English marl lake 2227

Keywords: eutrophication, macrophytes, marl lakes, palaeolimnology, pigments

capacity, following the predictions of regime shift and Introduction alternative stable state hypotheses (Scheffer et al., 1993; Marl lakes are calcite-depositing, high-alkalinity lakes, Scheffer & van Nes, 2007; Blindow, Hargeby & Hilt, globally distributed in areas of carbonate geology. They 2014). Despite prevailing theory suggesting resistance of are distinct from other lake types owing to their particu- Chara lakes to eutrophication, empirical evidence sug- larly clear, blue-green water, white calcareous sedi- gests that the macrophyte communities of marl lakes ments, and remarkably high macrophyte colonisation may actually be highly sensitive to nutrient enrichment depths (>10 m). Further, the macrophyte community (Wiik et al., 2013). For example, substantial declines in composition of marl lakes, consisting of a diversity of charophyte stands have been associated with total phos- À charophyte and Potamogeton species (Palmer, Bell & But- phorus (TP) exceeding only 7 lgL 1 (Free et al., 2007) À1 terﬁeld, 1992; Duigan, Kovach & Palmer, 2007), is recog- and nitrate-N (NO3-N) exceeding 2 mg L (Lambert & nised in the European Union Habitats and Species Davy, 2011). Further, shifts between clear and turbid Directive (EC-DG ENV, 2007), awarding marl lakes spe- conditions have occurred at relatively low TP concentra- À cial protection as a habitat. However, wherever marl tions (from c.20to70lgL 1, respectively) (Hargeby, lakes are found, their characteristic macrophyte species Blindow & Andersson, 2007). However, owing to the are declining, while taxa tolerant of human impact, espe- high biomass attained by charophytes, substantial cially eutrophication, are becoming more abundant amounts of nutrients can be locked into the benthos (Sand-Jensen et al., 2000; Kłosowski, Tomaszewicz & (Pełechaty et al., 2013; Pukacz, Pełechaty & Frankowski, Tomaszewicz, 2006; Baastrup-Spohr et al., 2013). Conse- 2014), detracting from the relevance of pelagic measures quently, concerns over the ecological quality of marl of eutrophication (e.g. TP, chlorophyll a). Substantial lakes have been raised (Blazen cic et al., 2006; Pentecost, ecological degradation in the benthos may occur prior 2009; Azzella et al., 2013). Eutrophication effects, such as to any increases in planktonic production. increased phytoplankton production, associated reduced It seems likely that the overwhelming majority of marl water transparency, as well as low sediment cohesion lakes have been impacted by human activity (Jeppesen, (Egertson, Kopaska & Downing, 2004; Schutten, Dainty Jensen & Søndergaard, 2002; James et al., 2005; Bennion & Davy, 2005) can be particularly damaging to marl et al., 2011), while minimally impacted sites tend to lie lakes because they restrict macrophyte colonisation in inaccessible, less researched areas (Blazen cic et al., depth and induce an upslope retreat of charophytes and 2006), leaving knowledge gaps at the early stages of Potamogetonaceae. The retreat in turn compresses plant eutrophication where more subtle, yet signiﬁcant, com- communities into a much narrower depth range and munity responses may occur. Palaeolimnological analy- thereby reduces species diversity (Middelboe & Mark- ses can provide a means to estimate pristine conditions, ager, 1997; Penning et al., 2008). and also detect changes in marl lakes over the decadal– Traditionally, marl lakes have been considered resili- centennial timescales relevant to long-term eutrophica- ent to eutrophication owing to the precipitation of phos- tion impacts, providing potentially more ecologically phorus with calcite (coprecipitation) (Otsuki & Wetzel, meaningful information than extrapolation from extant 1972; House, 1990; Robertson et al., 2007). Calcite deposi- analogue sites (Osborne & Moss, 1977; Moss, 1979; Sayer tion occurs predominantly in summer when photosyn- et al., 2010a). thesis increases pH and when water temperatures are Our study aimed to establish the early ecological relatively high, both of which induce carbonate oversat- changes that occur in marl lakes as a response to minor uration (Brunskill, 1969; Murphy, Hall & Yesaki, 1983). nutrient enrichment. We applied a combination of his- Therefore, macrophytes (especially charophytes) can pre- torical investigations, limnological monitoring (2009– vent phytoplankton dominance and maintain clearwater 2010) and palaeolimnology to Hawes Water (Lancashire, conditions via recycling of sediment-bound nutrients, U.K.) which currently has moderate limnetic phosphorus and inducing coprecipitation in the water column. The and chlorophyll a concentrations. Hawes Water has pre- strong negative feedback exerted by charophytes on viously been described as the ‘best example of a lowland external nutrient loading may lead to threshold marl lake in England’ (Bennion et al., 2009) and is responses once the latter exceeds their buffering classed as oligotrophic (Skelcher, 2014). We hypothesised

Fig. 1 The location, bathymetric profile and coring sites of Hawes Water. that intensification of agricultural practices and nitrogen mesotrophic waters with benthic vegetation of Chara deposition since the 1800s (McGowan et al., 2012; Moor- spp’ under the European Union Habitats and Species house et al., 2014) and especially since the mid-1900s Directive (EC-DG ENV, 2007). The catchment encom- (Vickery et al., 2001; Robinson & Sutherland, 2002) in passes 1.7 km2 and consists, in addition to the nature north-west England (Baddeley, Thompson & Lee, 1994; reserve, of holiday lets with private sewage manage- Pitcairn, Fowler & Grace, 1995) have resulted in major ment, and pasture. Annual P loads are estimated at ecological shifts in Hawes Water, associated with 51.27 kg, and the retention time is c. 0.32 years (Gold- upslope macrophyte movement and changing biological smith et al., 2003). community composition. Hawes Water has a long history of autochthonous carbonate precipitation extending through the Late Glacial to the present day (Marshall et al., 2002). During the Methods early Holocene, the maximum extent of the lake encom- passed c. 1 km in length and 400 m in width; however, Study site changes in sea level and therefore water table depth = Hawes Water is a small (5.7 ha) and shallow (Zmax gradually lowered water levels of the lake (Jones et al., = 12.2 m, Zmean 4.2 m) mesotrophic (mean annual TP 2011). Marginal Chara marl deposits were incised follow- À À 20 lgL 1 and chlorophyll a (chl a)12lgL 1) kettlehole ing late-Holocene reductions in water levels, exposing a lake in Silverdale, Lancashire, U.K. (54.11N 2.49W; terrestrialised marl bench (Jones et al., 2011). Fig. 1). It lies in a shallow basin of Carboniferous lime- Hawes Water is naturally spring and groundwater stone in Gait Barrows National Nature Reserve. The lake fed; however, an artificial inflow and outflow were cre- is within a Special Area of Conservation and is desig- ated in the 1800s, which lowered lake water levels and nated as an example of habitat 3140 ‘hard oligo- connected Hawes Water to Little Hawes Water within a

© 2015 The Authors Freshwater Biology Published by John Wiley & Sons Ltd., Freshwater Biology, 60, 2226–2247 Evidence for eutrophication in a low-nutrient English marl lake 2229 pasture to the north. The land surrounding the lake was 140 mm) ‘Big Ben’ piston corer (Patmore et al., 2014). subsequently improved from fen to arable. Water levels The site is a vegetated subsurface mound (known as the increased and fen development recommenced following ‘Chara mound’) separated from the lake margins by dee- discontinued maintenance of drainage ditches in the per open water habitat. A deep-water core HAWE5 early 1900s (Oldfield, 1960) until clearance again in the (31 cm) was taken in January 2011 from the central- 1960s. Reed swamp is currently developing in the mar- northern end of the lake at a water depth of 10 m using gins as a result of renaturalisation. Given the steeply a Glew corer (internal diameter 40 mm) (Glew, 1991). shelving bathymetric profile of the lake, areas of shallow Both cores were extruded at 1-cm intervals. water are restricted to the lake margins (Fig. 1). The limnology of Hawes Water is more thoroughly Core chronology and lithostratigraphy reported in Wiik et al. (2014). Briefly, peaks in pH occur during the summer months when marl precipitates on Freeze-dried sediments from cores HAWE3 and HAWE5 macrophytes (Nuphar lutea, Potamogeton coloratus, Pota- were dated through analysis of 210Pb, 226Ra, 137Cs and mogeton lucens, Chara spp.) leading to a decline in alka- 241Am by direct gamma assay in the Bloomsbury Envi- linity and very low or undetectable concentrations of ronmental Isotope Facility (BEIF) at University College soluble nutrients. Maximum Secchi depth (>5 m) occurs London. The absolute efficiencies of the detector were during the winter, and minima of ~1 m coincide with determined using calibrated sources and sediment sam- À peaks in chl a concentrations (28 lgL 1). TP peaks ples of known activity. Corrections were made for the À occur in winter (max 40 lgL 1), and soluble reactive P effect of self-absorption of low-energy gamma rays À (SRP) is constantly low (<5 lgL 1). Thermal and chemi- within the sample (Appleby, Richardson & Nolan, 1992). cal stratification begins in May/June and ends in 210Pb dates were calculated using the constant rate of September, with a thermocline at c.5–6 m. Subsurface supply (CRS) model (Appleby, 2001). Carbonate and oxygen maxima develop in late spring/early summer. organic carbon contents of HAWE3 and HAWE5 were quantified by loss on ignition (LOI) following Dean (1974). Contemporary macrophyte surveys

The macrophyte community and sediment type were Pigments monitored semi-quantitatively by standard shore and boat surveys (JNCC, 2005) using a bathyscope and a Chlorophyll and carotenoid pigments were analysed at rake to determine submerged taxa. Shore surveys were all levels of the HAWE3 core following McGowan et al. 80 m long and extended to water depths ranging from (2012). Freeze-dried samples were extracted overnight at 25 to >75 cm. Boat surveys departed from the centre of À4 °C in a mixture of acetone, methanol and water the shore section, and multiple points were recorded up (80 : 15 : 5). Extracts were filtered with a 0.22-lm PTFE to the maximum depth of macrophyte colonisation. The filter, dried under N2 gas and redissolved in a 70 : 25 : 5 maximum depth of colonisation was assessed with sev- mixture of acetone, ion pairing reagent (IPR 0.75 g tetra- eral rake throws. Four sections were selected in total, butyl ammonium acetate and 7.7 g ammonium acetate and additional littoral areas were thoroughly surveyed in 100 mL water) and methanol for injection into an Agi- in 2010 in an attempt to find rare charophyte species. lent 1200 series high-performance liquid chromatogra- Macrophytes were identified to genus or species level in phy (HPLC) unit with separation conditions modified the field excepting charophytes which were sent for from Chen et al. (2001). identification to Nick Stewart (recorder of charophytes for the Botanical Society of Britain and Ireland). Charo- Macrofossils phyte nomenclature followed Bryant & Stewart (2002). On a whole-lake scale, macrofossils of single sediment cores have been shown to reflect changes in the domi- Core collection _ nant component of biological communities (LevI et al., Two sediment cores were taken from Hawes Water to 2014; Davidson et al., 2005; Zhao et al., 2006). Macrofos- capture ecological changes in the littoral and pelagic sils, representing macrophyte, cladoceran, trichopteran, zone (Fig. 1). A littoral core (HAWE3, 71 cm) was taken algal, bryozoan and molluscan communities were enu- in October 2009 at a depth of 4.1 m at the northern end merated every 4 cm for HAWE3 and every 6 cm for of the lake using a wide-bore (internal diameter HAWE5. Approximately 30 cm3 of sediment per sample

© 2015 The Authors Freshwater Biology Published by John Wiley & Sons Ltd., Freshwater Biology, 60, 2226–2247 2230 E. Wiik et al. was used for analysis of both cores. Samples were netic changes, indicated by profiles of fucoxanthin and soaked in 5% KOH overnight and sieved through chlorophylls + chlorophyll derivatives. Lutein (chloro- meshes of 125 and 355 lm (HAWE3) and 250 lm phyte and higher plant biomarker) coeluted with zeax- (HAWE5). All material of the larger size fractions, and a anthin (cyanobacterial biomarker), and so the two are subsample of c. 15% of the 125 lm size fraction, was reported together. analysed. Macrofossil counts were expressed as numbers The absolute abundances of individual pigments (Leav- per 100 cm3. Abundance scores between 0 and 3 (high itt, 1993) and macrofossil remains (Levine & Schindler, numbers indicate high abundance) were used for 1988) in a core profile may not accurately portray their rel- Chara spp. stem encrustations (HAWE3), and for bryo- ative abundance as part of a composition owing to differ- phyte, mollusc and Nymphaeaceae trichosclereid (leaf ential preservation and/or production. In order to cell) remains (HAWE5). dampen the effect of non-comparable abundances, and to Where species-level detail was not attainable, remains stabilise variance, both data sets were log-transformed were aggregated to genus or higher level. This and standardised prior to analysis. Ecological distances includes Daphnia hyalina agg. ephippia (HAWE3) (U.K. were measured using Bray–Curtis dissimilarities given species other than D. magna and D. pulex), Daphnia spp. the suitability of this metric for data sets with numerous ephippia (HAWE5) (U.K. species other than D. magna), zero values (Beals, 1984). Bray–Curtis dissimilarities can- Potamogeton pusillus agg. leaf tips (P. pusillus and P. ber- not be mapped using PCA owing to the data structure chtoldii) and Nymphaeaceae trichosclereids (Nymphaea requirements of the latter (Legendre & Gallagher, 2001), alba and Nuphar lutea). Molluscs were mostly identified and therefore, ordination was performed using non-met- to family or genus level. Oospores are morphologically ric multidimensional scaling (nMDS). Core zonation was highly plastic and were therefore aggregated as Chara determined by clustering using Ward linkage. spp. for numerical purposes. However, oospore morpho- The similarity between community changes in data types were also tentatively identified using reference col- sets, as indicated by ecological distance matrices, can be lection material and an oospore key (Haas, 1994). assessed by graphical comparison of their ordinations. Uncalcified and calcified oospores were counted sepa- In Procrustes analysis, two ordinations are rotated and rately. scaled in order to maximise their fit onto each other, Trichoptera frontoclypea were identified by Malcolm which has been shown to be robust for the analysis of Greenwood and Paul Wood at the Department of Geog- ecological data sets (Peres-Neto & Jackson, 2001). The raphy, Loughborough University, and moss remains, by similarity between community change in the pigment Graeme Swindles at the Department of Geography, and macrofossil data was assessed with symmetric University of Leeds and Pauline Lang at the Scottish nMDS-based Procrustes analysis for matching strati- Environment Protection Agency. graphic levels (n=17), using the R PROTEST function to test significance and correlation (Peres-Neto & Jack- son, 2001). Historical survey records Diagrammatic reconstructions of macrophyte commu- Historical macrophyte records were assembled from nities and colonisation depths in Hawes Water for the Natural England archives, survey data held at UCL, old present, the mid-1900s and the late 1800s/early 1900s scientific publications and reports, and through personal were informed by a combination of the following communication with scientists and field naturalists sources of information: (i) palaeolimnological analyses in familiar with the site. Unpublished limnological moni- this study; (ii) historical survey data on macrophyte toring data were provided by J. Marshall. presence and distribution encompassing the early 1900s to the present; (iii) macrophyte surveys conducted in this study in 2009 and 2010; (iv) data for macrophyte Data analysis community composition, depth zonation and maximum Numerical analysis was performed on HAWE3 using colonisation depth in comparable marl lakes (Jupp, the statistical software R version 2.1.2 (R Development Spence & Britton, 1974; Spence, Barclay & Allen, 1984; Core Team, 2010) with the packages analogue (Simpson, Pentecost, 2009; Hilt et al., 2010) or lakes with similar 2007; Simpson & Oksanen, 2011) and vegan (Oksanen species assemblages (Spence, 1967, 1982); and (v) ecolog- et al., 2011). ical literature (morphology, habitat, distribution) on Bri- The uppermost two core levels of the pigment data tish charophytes (Stewart & Church, 1992; Moore, 2005) set were omitted from analysis given their large diage- and Potamogetonaceae (Preston, 1995).

© 2015 The Authors Freshwater Biology Published by John Wiley & Sons Ltd., Freshwater Biology, 60, 2226–2247 Evidence for eutrophication in a low-nutrient English marl lake 2231 Several species existed as occasional patches includ- Results ing Elodea canadensis (found only once in 2009), Pota- Macrophyte communities in 2009 and 2010 mogeton lucens (recorded <4 m depth along the southern margins), Nymphaea alba, Hippuris vulgaris (in the west Similar macrophyte communities were observed in the and north) and an unidentified ornamental Nymphaea 2009 and 2010 surveys in Hawes Water. The margins cultivar. The distribution of Myriophyllum spicatum was (<75 cm water depth) were densely vegetated restricted to a subsurface mound (4 m depth; the coring with Phragmites australis, Typha angustifolia, Schoenoplec- site) off the north shore where N. lutea was dominant. tus lacustris and Cladium mariscus (85–90% of total marginal habitat (TM)). Salix spp. were occasional in the shallowest areas (<5% TM). In open water patches Historical macrophyte communities within the emergent vegetation to the north, east and Seven historical macrophyte survey data points were south, Utricularia sp., Nuphar lutea and Potamogeton col- obtained, two from the beginning of the 20th century oratus were locally abundant (c. 5% TM). Chara virgata, (1911, 1915), one from the mid-1900s (1969) and four Chara aspera and Chara contraria were very scarce (<5% from the end of the century (1982, 1993, 1995, 1999). TM). In 2010, filamentous algal growth was abundant in These survey data are not floristically complete (i.e. all the reedswamp, and sediment in the margins was species present not recorded) and therefore provide spe- unconsolidated. Fontinalis antipyretica was locally fre- cies presence data only. Changes in Chara nomenclature quent (<5% TM) in the shallower water, and Lemna and taxonomy (Bryant, Stewart & Stace, 2002) challenge minor as well as Lemna trisulca occurred in the north-east historical interpretations. For example, C. curta and margins close to the inflow. C. aspera were recorded separately in 1999, but these can Emergent vegetation became less dense with increas- also be synonymous depending on naming authorities, ing water depth. Below c. 75 cm, P. coloratus remained which were unfortunately not always available for these locally abundant, and N. lutea became dominant, with data. C. globularis (last recorded 1915) may also refer to submerged mats of Chara aculeolata covering marl C. virgata (recorded in 2009/2010), and therefore, the for- shelves mainly to depths of c. 2 m (but up to 3.9 m in mer may still be present in the lake. an area off the regular transects in 2010). In deeper Historical records suggest that several species have water, communities were dominated by N. lutea which disappeared from the lake throughout the 1900s (last was the deepest coloniser. Open water habitat covered record in parentheses) including Littorella uniflora (1911); most of the lake, as vegetation was restricted to depths Chara vulgaris var. papillata (1915); Chara globularis, Chara <5.5 m.

Table 1 Historical records of macrophytes in Hawes Water, and all species recorded in 2009/2010

Date Flora Source

2009 Chara aculeolata, Chara aspera, Chara contraria, Chara virgata, Fontinalis antipyretica, Cladium Surveys by author & -10 mariscus, Elodea canadensis, Hippuris vulgaris, Lemna minor, Lemna trisulca, Myriophyllum spicatum, Nuphar lutea, Nymphaea alba, Phragmites australis, Potamogeton coloratus, Potamogeton lucens, Schoenoplectus lacustris, Typha angustifolia, Utricularia vulgaris agg. 1999 C. hispida var. hispida (syn. C. aculeolata), C. hispida var. major; Chara rudis to 4 m depth; Survey by C. Newbold P. lucens 1995 C. aculeolata, C. aspera, Chara curta, C. hispida, C. rudis, P. lucens, Potamogeton natans 1996 Natural England report for Cumbria County Council and ARC 1993 Chara sp. recorded at HAWE3 coring site at depths around 4 m Field notes by J. Marshall 1982 Encrusted C. aspera abundant, C. rudis to 7 m depth; E. canadensis, P. lucens; ‘Water was Diver survey by C. Newbold et al. very brown in colour and visibility was poor.’ – M. Wade 1969 C. aculeolata, C. aspera, C. mariscus, N. lutea, N. alba, P. australis, Potamogeton crispus, School trip; J. Birks Potamogeton friesii, P. lucens, Potamogeton obtusifolius 1915 C. aculeolata, Chara fragilis subsp. delicatula (possibly syn. C. globularis), C. rudis C. vulgaris W. H. Pearsall, in Druce, (1916) var. papillata, P. friesii, P. lucens, P. obtusifolius, H. vulgaris, N. lutea, N. alba; ‘The Characeae are in considerable quantity and much encrusted’ 1911 A record of Littorella uniﬂora Druce (1911) 1850 The water is very clear Davis (1850); Dean & Jackson (1905)

© 2015 The Authors Freshwater Biology Published by John Wiley & Sons Ltd., Freshwater Biology, 60, 2226–2247 2232 E. Wiik et al. hispida var. hispida, Chara rudis (1999); Potamogeton cris- than HAWE3 (11 to 66%). The dating profile of HAWE5 pus (1969); Potamogeton friesii, Potamogeton obtusi- extended from 0 cm (2011) to 8.5 cm (1894) and reflected folius (1984); and Potamogeton natans (1995) (Table 1). much lower sedimentation rates in the profundal than The data, including comprehensive plant cover maps, in the littoral (Fig. 2b). Sedimentation rates increased À À suggest that species evenness (similarity in % cover over time, from <0.01 g cm 2 yr 1 in the first half of the À À between species) has reduced in the lake over the 20th 20th century to >0.01 g cm 2 yr 1 in the latter half. Sedi- century. For example, C. aculeolata and C. aspera notwith- mentation rate roughly doubled between the late 1990s À À standing, charophyte species recorded in 2009 were not and the 21st century (c. 0.015 to >0.03 g cm 2 yr 1). found in 2010 and it seems that any findings were chance occurrences of very scarce populations. C. rudis, Cross-comparison of core depth ages. The dating profile of previously recorded to depths of 7 m (1982), later 4 m HAWE5 (1894–2011) extended further back in time than (1999), is now absent, reflecting decreasing colonisation that of HAWE3 (1947–2009), which hindered compre- depths to the point of extinction. The northern mound hensive cross-comparison (Fig. 2a,b). Owing to the fluc- currently hosting N. lutea and M. spicatum only was, tuation of sedimentation rates where the dating record during a coring trip in 1993, described as ‘mostly Chara’ of HAWE3 terminated (16.5 cm), extrapolation of sedi- and named the Chara mound (J. Marshall, field notes). ment age further downcore was not possible. Instead, In contrast, it was noted in 1915 that ‘The Characeae are approximate aging was undertaken using the similarities in considerable quantity and much encrusted’ (Table 1). between the macrofossil record of both cores, namely Hippuris vulgaris was recorded, in addition to the west abrupt changes in the amount of terrestrial wetland fringe, at the north and south ends of the lake in 1982 material, and of Juncus seed and Chara oospore abun- (noted as ‘frequent’ (F) on the DAFOR scale), and the dance, aged around the early 1800s in HAWE5. These north end in 1999, showing that the distribution of the changes occurred at 48.5 cm in HAWE3. species has diminished. This is also the case with P. lucens, and E. canadensis, also recorded as F in 1982. Utric- Stratigraphic clusters and community change ularia vulgaris agg., on the other hand, has increased in abundance, recorded as ‘rare’ in 1982 and currently Four major stratigraphic zones were identified in the abundant throughout the lake margins. HAWE3 pigment and macrofossil data through cluster analysis (Fig. 3a,b). Although comparison of the clusters between the proxies was restricted by the lower resolu- Core chronologies and geochemical characteristics tion of the macrofossil data, similar groupings were evi- HAWE3. Core HAWE3 comprised a mixture of marl- dent particularly for the core base and top (Fig. 3a,b). coated macrophyte stem encrustations and organic mate- The lowermost clusters consisted of base to 50.5 cm for rial, with considerable fluctuation in composition pigments, and base to 48.5 cm for macrofossils (pre- throughout (Fig. 2a). Overall, carbonate content was 1800s), and the top clusters of 12.5 (1960s) to the core high (48 to 57%), low values coinciding with darker top for both data sets. Discrepancies in clusters for the brown sediment between 43 and 10 cm. Organic matter middle section of the core (pigments: 49.5–37.5 cm;36.5– fluctuated inversely to carbonate and was relatively low 13.5 cm; macrofossils: 44.5–28.5 cm, 24.5 –16.5 cm) indi- (4 to 13%). The dating profile of HAWE3 extended from cated an earlier change in pigments (phytoplankton, 0 cm (2009) to 16.5 cm (1947), with sediment accumula- epiphytes, macrophytes) compared with macrofossils À À tion rates fluctuating around 0.077 g cm 2 yr 1 for the (macrophytes, molluscs, cladocerans, bryozoans). last sixty years (Fig. 2a). Further, cluster analysis showed pigment samples 49.5–37.5 cm to be more similar to the upper core (from HAWE5. HAWE5 was characterised by much finer sed- 12.5 to 2.5 cm) than to levels between 36.5 and 13.5 cm. iment than HAWE3 and lacked encrusted stem remains. This was reflected by a relatively large ecological dis- Relatively high organic content at the core base (Fig. 2b) tance between 36.5 and 13.5 cm and all other clusters in was reflected by a dark brown/black sediment colour. ordination space (Fig. 3b). For macrofossils, ecological Carbonate content increased upcore to 24.5 cm, ranging change was more monotonic in time, with 44.5–28.5 cm from 13 to 44%. At 24.5 cm, carbonate content stabilised in one cluster and 24.5–16.5 cm and 12.5–0.5 cm in and fluctuated only slightly between 39 and 50% around another (Fig. 3a). a mean value of 43%, with slightly lower concentrations There was a very high degree of concordance in the above 11.5 cm. Organic matter was generally higher patterns of temporal change between the pigment and

(a) –2 –1 % CO3 % organic matter Error (±year) in age (year AD) SR (g cm year )

0 2009 2007 1997 1986 1980 10 1974 1971 1963 1956 1947 20

40 Depth (cm)

48 50 52 54 56 4 6 8 10 12 0 2 4 6 8 10 12 0.04 0.06 0.08 0.10

(b) –2 –1 % CO3 % organic matter Error (±year) in age (year AD) SR (g cm year )

0 20112009 1999 1990 1979 5 1966 1947 1923 1894 10

15 Depth (cm) 20

Fig. 2 Loss-on-ignition and sedimenta- 30 tion rate (SR) for cores HAWE3 (a) and HAWE5 (b), with 210Pb-derived dates 20 30 40 50 10 20 30 40 50 60 0 5 10 15 20 with errors. 0.0050.0100.0150.0200.0250.030 macrofossil data, reflected by the high PROTEST correla- trophic Sphagnum austinii in particular). DOC influx to tion of 0.77 (P < 0.002). There was no strong pattern the lake was also indicated by a strong amber and black between stratigraphic position and residuals; however, colour dissolved from the sediment during processing. slightly less agreement was apparent within the lower- Pockmarked mollusc shells, rounded marl agglomer- most 30 cm (Fig. 3c). Particularly strong concordance ations and the absence of well-preserved encrusted was evident for samples at 68.5, 36.5 and 0.5 and poor Chara stem remains indicated diagenetic dissolution of for samples at 64.5, 52.5 and 44.5. carbonate structures. Aquatic remains included numerous calcified and uncalcified Chara oospores and leaves of hypnoid Zone 1: pre-1800s mosses cf. Platyhypnidium riparioides (Figs 5 & 6). Fur- The concentration of pigments, including ubiquitous ther, a small number of Potamogeton coloratus seeds pigments (chl a, pheophytin a and b-carotene), was very (56.5 cm, 52.5 cm) and one seed of Potamogeton cf. perfo- low in zone 1 (Fig. 4). Pigment profiles of chlorophylls liatus (56.5 cm) were found (Fig. 5). Leaf cells of Nym- and their degradation products (not shown) indicated phaeaceae were relatively low in abundance (Figs 5 & stable preservation conditions over time. 6); a small number of Nymphaea alba seed fragments Abundant remains of terrestrial wetland taxa occurred (23.5 cm), P. berchtoldii/pusillus leaf tips (11.5 cm) and a in zone 1, including Juncus spp. seeds, and leaves of marl-encrusted M. spicatum turion (23.5 cm) were found Sphagnum subsection acutifolia (those of the ombro- in HAWE5.

(a)Macrofossils (b) Pigments

46.5 28.5 ● ●

● ●

● 0.1 0.2 48.5

0.1 0.2 0.3 ● 36.5 ● 49.5 ● ●● ● 50.5 24.5 ● ●●●● ● ● ● ●●●● 37.5 ● ● ● ● ● ● ●● ● ●

0.0 1950s 0.0 8.5 ●● 13.5 ● ●● ● 1980s NMDS2 ● NMDS2 ● 1940s Core base 1960s 16.5 12.5 ~2000 Core base 2.5 1960s 12.5 −0.2 −0.1 −0.3 −0.2 −0.1 −0.3 −0.2 −0.1 0.0 0.1 0.2 −0.2 −0.1 0.0 0.1 0.2 0.3 0.4 0.5 NMDS1 NMDS1 m 12 = 0.40 P = 0.002 Residuals (c) r = 0.78 Procrustes errors 28.5 48.5 0.050.100.150.20 44.5 Depth (cm) 40.5 60.5

32.5 64.5 10 36.5 20.5 68.5 20 Fig. 3 nMDS (a, b) and Procrustes analy- 24.5 52.5 sis (c) of HAWE3 macrofossil and pig- 16.5 56.5 30 ment data. Core levels (c), and where Dimension 2 available, dates as years (a, b), are shown 3.5 40 between the core base and top where lar- 12.5 ger community shifts take place. nMDS −0.2 −0.18.5 0.0 0.1 50 symbols represent different clusters; solid −0.2 −0.1 0.0 0.1 0.2 0.3 0.4 60 lines, a major cluster division; and Dimension 1 dashed lines, subordinate cluster divi- 70 sions.

Molluscs (Sphaeriidae, Radix cf. peregra and Bithy- slow-flowing water (Wallace, Wallace & Philipson, 2003; nia spp.) were present in this section, with a slight Edington & Hildrew, 2005). decrease in abundance at 52.5–48.5 cm (Fig. 7). In contrast, round and oblong morphotypes of Plumatella stato- Zone 2: pre-1800s/1800s blasts increased c. fourfold between these two core levels. No ephippia of pelagic cladocerans were Most pigments increased markedly in zone 2, especially recorded in HAWE3, but very scarce remains were those from siliceous algae (diatoxanthin, fucoxanthin), found in HAWE5. Carapaces of the macrophyte-associ- chlorophytes (chl b, phaeophytin b) and cryptophytes ated Pseudochydorus globosus were relatively low in num- (alloxanthin) (Fig. 4). Cyanobacterial pigments did not ber. follow the same pattern; increases in myxoxanthophyll, Trichopteran frontoclypea were recorded infrequently canthaxanthin and lutein–zeaxanthin in this core section in this zone with the following species appearing in both were very modest, and aphanizophyll was only occa- cores: Limnephilus marmoratus, Ecnomus tenellus and Mys- sionally above detectable levels. tacides longicornis (data not shown). Further, HAWE3 Influx of terrestrial matter was reduced compared included Mesophylax impunctatus, Mystacides azurae and with zone 1 as indicated by only a slight yellow tint in Sericostoma personatum. HAWE5 included Oecetis lacus- the sample water, and lower abundances of terrestrial tris, Athripsodes aterrimus, Holocentropus piscicornis and remains (Juncus, Sphagnum). There was no evidence of Polycentropus irroratus. It is noteworthy that eight of the calcite dissolution in the macrofossil material, and well- ten species recorded from HAWE5 occurred at 23.5 cm, preserved Chara stems were abundant throughout this six exclusively in this level. M. impunctatus, S. person- core section. In contrast, the abundance of oospores was atum and Polycentropus flavomaculatus are species associ- markedly lower (100s) compared with zone 1 (1000s) ated with stony, exposed surfaces. The other species are (Fig. 5) and no seeds of Potamogetonaceae or Jun- not exclusively found in one habitat, although all afore- cus were recorded. The abundance of Nymphaeaceae mentioned species are generally associated with still or remains was similar to the earlier core section.

All plants/algae Chlorophytes Cryptophytes

Year (AD) 2009 Beta−carotene Chlorophyll a Pheophytin a Chlorophyll a' Chlorophyll b Pheophytin b Alloxanthin Zones 0 0 1980s 4 1960s 10 10 1940s 20 20 3 30 30

40 40 2 <1800 50 50 60 60 1 70 70

0 50 100 150 200 250 3000 204060801000 50 100 150 200 250 0 10203040500 20406080 0 200 400 600 800 0 102030405060

Depth (cm) Cyanobacteria ...... + Chlorophytes Diatoms Siliceous algae

Year (AD) Zones 2009 Echinenone Myxoxanthophyll Aphanizophyll Canthaxanthin Zeaxanthin + Lutein Diatoxanthin Fucoxanthin 1980s 4 1960s 1940s 3

2 <1800

050100150051015200 100 200 300 400 500 0 100 200 300 400 500 0 500 1000 1500 0 204060 0510 − nmol g 1 organic matter

Fig. 4 HAWE3 pigment data ordered by group. Solid lines demarcate stratigraphic clusters.

) d e e s ( s tu a li oospore oospore (seed) fo stem r e .p Chara Potamogeton.f Year (AD) Depth (cm)Chara (calcified) c Potamogeton coloratusChara Nymphaeaceae Zones (uncalcified) Juncus (seed) encrustation score (trichosclereid) 2009 4 1970s 10 1940s 20 3 30 2 40 <1800 50

60 1

0 0 1 0 0 0 2 0 25 0 16 16 400 1600 3600 6400 2500 10 000 22 500 Counts as n (individuals)100 cm–3 wet sediment

Fig. 5 Plant macrofossil data from HAWE3. Solid lines demarcate stratigraphic clusters.

Among the mollusc remains, those of Bithynia spp. Plumatella statoblasts, followed upcore to 28.5 cm by an and the Sphaeriidae were found in lower abundances equally dramatic decrease. Remains of P. globosus were than in zone 1, and the remaining taxa did not display more abundant than in zone 1, and no pelagic clado- changes (Fig. 7). Between 48.5 and 40.5 cm, there was ceran taxa were recorded. Trichopteran head shields in a fourfold increase in the abundance of oblong this core section were largely absent (n = 3) and those

htoldii

erc

eaceae hyalina s (seed) a sc ni u riophyllum spicatum ympha Nymphaea alba Moss Moll pusillus (leaf) N Daph Daphnia pulex Year (AD) Depth (cm) Juncus (seed fragment) My (turion) Charophyte(uncalcified) oospore Potamogeton b (trichosclereid) (ephippium) (ephippium) Zones 2011 1990s 1970s 5 1940s Post-1800s 1890s 10

20 Pre-1800s

0 0 1 2 3 0 1 2 3 0 1 2 3 0 0 1 2 3 0 1 2 3 0 0 1 2 3 20 40 60 20 40 1.0 1.5 2.0 2.5 3.0 20 40 Mosses, Nymphaeaceae and molluscs: 0−3 abundance scale Others: n (individuals) 100 cm–3 sediment

Fig. 6 Macrofossil data from HAWE5. Owing to differences in the dating profiles and data resolution between HAWE5 and HAWE3, the core zonation of HAWE3 was not applied to HAWE5. The solid line shows the change in macrofossil composition matched with HAWE3 and identified as the period of hydrological alteration. found were towards the base of the section, identified as encrustations remained abundant in HAWE3, and M. longicornis, A. aterrimus and P. irroratus (data not P. pusillus/berchtoldii leaf tips were numerous in HAWE5 shown). These taxa show broad habitat preferences (Fig. 6). Based on fossil evidence, historical photographs including sand, mud and vegetation (Wallace et al., and macrophyte survey data (Table 1), macrophyte 2003; Edington & Hildrew, 2005). colonisation depth decreased, macrophyte abundance increased, and community composition became more mesotrophic towards the latter period of zone 3 (Fig. 8). Zone 3: 1800s/1900s to the late 1960s Within the invertebrate community, the beginning of Pigments from cyanobacteria (echinenone, aphanizo- zone 3 was marked by a relatively large increase phyll and myxoxanthophyll), including potentially nitro- in Bithynia spp., and a fourfold decrease in the abun- gen-fixing taxa (Hertzberg & Liaaen-Jensen, 1971), dance of oblong Plumatella statoblasts (Figs 5 & 7). Fur- siliceous algae (diatoxanthin) and all algae (b-carotene), ther upcore, the abundances of the aforementioned taxa increased steeply to 25.5 cm and displayed a variable, were relatively stable. Two Trichoptera species were but decreasing trend, upcore. Concentrations of alloxan- recorded, P. flavomaculatus (16.5 cm; 1940s) and Cyrnus thin were relatively stable in this core section, and flavidus (20.5, 16.5 cm; c. 1930s-1940s), the latter appear- fucoxanthin varied with no trend. Ubiquitous pigments ing in the core record for the first time. C. flavidus is a (chl a, phaeophytin a, b-carotene) increased between 36.5 species found in a variety of habitats, especially among and 25.5 cm, levelling off thereafter and increasing in macrophytes, in still water. variability. In the upper end of the zone (16.5–13.5 cm; 1940s–1960s), there was a distinct peak in the chloro- Zone 4: late 1960s–early 2000s phylls. No changes in pigment preservation were indicated by chl a: degradation product values (not shown). The transition to zone 4 marks a succession in cyanobac- The transition from zone 2 to zone 3 (28.5–24.5 cm) in terial pigments (Fig. 4). Aphanizophyll and myxoxantho- HAWE3 marked a small increase in Nymphaeaceae phyll declined to undetectable concentrations, and remains, mirrored in HAWE5 (Figs 5 & 7). Chara stem echinenone to relatively low concentrations, coincident

Mollusca Bryozoa Cladocera

peus) m) bosus + leachi −cly (ephippium)ippiu g) s glo und) lon sp. peregra (eph + leachi . sp. sp. cf. t: ob lla phalus las ptera (fronto

Year (AD) Depth (cm) Sphaeriidae Valvatidae + Planorbidae Lymnaea Bithynia(operculum) tentaculata Bithyniatentaculata Plumate(statoblast: ro Plumatella(statob Tricho OribatidaPseudochydoru(carapace)Simoce Daphnia sp Zones 2009

1980s 4 1970s 10

1940s 20 3

2 40

<1800 50

60 1

0 0 0 0 0 0 0 0 0 0 5 9 25 25 25 25 25 25 16 25 25 100 100 225 400 625 100 225 100 225400 100 225 400 625 100 225 400 625 100 400 900 100 100 1600 2500 Counts as n (individuals) 100 cm–3 wet sediment

Fig. 7 Animal and bryozoan macrofossil data from HAWE3. Solid lines demarcate stratigraphic clusters. with sharp increases in canthaxanthin and lutein–zeaxan- and Planorbidae in particular (Fig. 7). The Plu- thin, which occur primarily in cyanobacteria that do not matella statoblast morphotypes were recorded only occa- fix nitrogen (Hertzberg & Liaaen-Jensen, 1971; Steenber- sionally. Cladocerans (Simocephalus sp., P. globosus) gen, Korthals & Dobrynin, 1994). Chlorophylls a and b increased in abundance between 16.5 and 12.5 cm decreased to concentrations similar to those preceding the (1940s–1960s) and then displayed little variability to the distinct peak around 14.5 cm (1950s). b-carotene, fucoxan- core top. The first records of Daphnia spp. occurred in thin, diatoxanthin and alloxanthin did not change notice- the uppermost samples of both cores (Figs 5 & 6). The ably in the uppermost 12.5 cm (post-1970s). only trichopteran species recorded in this zone was Macrofossils in zone 4 showed little change. C. flavidus. Chara stems remained abundant, oospores remained low in abundance, and no seeds of Potamogetonaceae were recorded (Fig. 5). However, there was a large and steady Discussion increase in Nymphaeaceae trichosclereids towards the Hydrological change versus eutrophication core top of HAWE3, the uppermost sample (2000s) con- taining 10 times more trichosclereids than the previous Understanding ecological responses to eutrophication level (3.5 cm; late 1990s). A similar increase occurred in requires that the effects of other impacts such as hydro- HAWE5. Compared with macrophyte community com- logical alteration are differentiated because they can position in the earlier 1900s (Fig. 8, Table 1), reduced have similar effects on the structure of benthic and pela- diversity of intermediate-depth macrophyte species, gic habitats (Hannon & Gaillard, 1997; Luoto et al., reduced macrophyte colonisation depth, and dominance 2011). However, land improvement in catchments prone of Nymphaeaceae, characterised the latter stages of zone to water logging such as around Hawes Water often 4 (Fig. 8). superimposes change in drainage on the top of increased Molluscs showed a slight decrease in abundance nutrient export (Skaggs, Breve & Gilliam, 1994; Snyder towards the core top, Lymnaea peregra and the Valvatidae & Morace, 1997; Dils & Heathwaite, 1999), creating

sp. & spp.

Salix PhragmitesCladium australisChara mariscus TyphaSchoenoplectus angustifoliaUtriculariaPotamogeton vulgaris lacustrisChara NupharaculeolataPotamogeton lutea Myriophyllum Alnus coloratus lucens spicatum

Present 2m 5–6 m 4.3 m 12 m

agg. lacustris

sp. & spp.

Salix Chara UtriculariaCladium vulgarisPhragmites mariscus Typha australisSchoenoplectus angustifolia PotamogetonChara Potamogeton aculeolata coloratusPotamogeton crispusNuphar lutea Potamogeton Potamogeton Chara rudis Potamogeton Alnus perfoliatus berchtoldii obtusifolius lucens

Potamogeton friesii

1960s Potamogeton coloratus 5–6 m 4.3 m 12 m

Chara rudis, Chara globularis, Chara contraria

lacustris

sp. & spp. Chara sp. ?

Salix PhragmitesChara australis Littorella Cladium uniflora mariscusTypha SchoenoplectusangustifoliaPotamogeton Chara aculeolataNuphar lutea PotamogetonChara rudis Potamogeton PotamogetonPotamogeton Alnus coloratus berchtoldii obtusifolius lucens friesii

Late 1800s/ 5–6 m 4.34.3m4.3 m 12 m early 1900s

Chara rudis, Chara globularis, Chara contraria

Fig. 8 Cross-sections of macrophyte community composition, showing the currently dominant community and historical communities based on available macrofossil and archival data. concurrence between the two and making it difﬁcult to In Hawes Water, the clearest evidence in the palae- disentangle individual effects. Further, a change in water olimnological record for point change (as opposed to source from exclusively ground water to ground and gradual and continuous) driven by water-level reduction surface water, as in Hawes Water, changes the relative was in the shift from zone 1 to zone 2 when inlets and and absolute lake P and N loads, given the predomi- outlets were created, characterised by large ecological nance of N in ground water and P in surface waters. distances between samples. This change resulted in

© 2015 The Authors Freshwater Biology Published by John Wiley & Sons Ltd., Freshwater Biology, 60, 2226–2247 Evidence for eutrophication in a low-nutrient English marl lake 2239 reduced influx of wetland soil and-macrofossils to the Berg et al., 1998), seen as a decline in charophyte diver- lake (in the macrofossil record) with the expansion of lit- sity and coverage; (ii) increases in primary production toral habitat and reduction of pigment degradation in in the water column (Schindler, 1978; Vadeboncoeur the water column (in the pigment record) (Leavitt, 1993; et al., 2003), seen as increases in limnetic chl a (E. H. Cuddington & Leavitt, 1999). Fisher, R. T. Jones, S. Barnes, S. F. Crowley & J. D. Mar- Following the creation of inlets and outlets, the pat- shall, unpubl. data; this study) and in fossil Daphnia tern of change in the core record suggests a predomi- spp. ephippia (Davidson et al., 2011); (iii) increases in nance of long-term change in the biological the proportion of eutrophic indicator taxa in the plank- compositions driven by eutrophication despite a second ton (Reynolds et al., 2002), seen as recent increases in the event of water level lowering around the 1960s (R. Petley- relative abundance of the diatoms Fragilaria crotonensis, Jones, pers. comm.). There are three factors that indicate Asterionella formosa and Stephanodiscus medius in sedi- this dominance of nutrient enrichment above zone 2. ment cores (Bennion, 2004) consistent with abundances Firstly, the fossil assemblages did not return to the of Stephanodiscus sp. in algal blooms (E. H. Fisher, R. T. compositions coincident with the first occurrence of Jones, S. Barnes, S. F. Crowley & J. D. Marshall, unpubl. water-level change, suggestive of a changing baseline. data); and (iv) reduced water transparency accompanied Secondly, no change was evident in pigment preservation by reductions in macrophyte colonisation depth (Mid- conditions such as might have indicated a discrete water- delboe & Markager, 1997), seen as the decrease in Chara level change. Thirdly, independent evidence of increasing rudis from 7 m (1982) to 4 m (1999) followed by its nutrient loads to the lake following this period included extinction from the lake. increasing (allochthonous) organic sedimentation following the 1970s (core HW1; J. Holmes, pers. comm.), high Timescales of change in biological community structure nutrient concentrations in the inflow, expansion of private and indications of state change sewage works and occasional slurry applications in the catchment (Goldsmith et al., 2003). Renaturalisation com- Scheffer & van Nes (2007) suggested that transitions bined with reed bed management at the downstream from macrophyte-dominated clearwater states to turbid Leighton Moss nature reserve has also introduced large phytoplankton-dominated states may occur as a gradual numbers of both migratory and non-migratory birds to process punctuated by more major shifts in community the area, which may have contributed to nutrient loads at composition. In marl lakes, where charophytes may Hawes Water. However, their potential nutrient impact maintain a particularly strong inertia to external nutrient could not be evaluated with available data. loading owing to their high biomass potential and hence ability to sequester nutrients, critical thresholds and periods of rapid ecological change may be expected Biological evidence of eutrophication in Hawes Water (Scheffer & van Nes, 2007; Blindow et al., 2014). Two The ‘baseline’ conditions in the pre-1800s, prior to points of subdecadal