Testing the light : nutrient hypothesis in an oligotrophic boreal lake

Anthropogenic changes in the nitrogen (N), phosphorus (P), and carbon (C) cycles have altered nutrient concentrations and the light climate in freshwaters globally. These factors affect phytoplankton (PPr) and bacterial production (BP), which constitute the basal energy resource for higher trophic levels in the pelagic zone of lakes. The light : nutrient hypothesis (LNH) predicts that although basal production decreases at low light, seston C : nutrient ratios also decrease, thus increasing food quality for crustacean zooplankton and potentially offsetting the negative effects of reduced food availability. We tested the LNH in an oligotrophic boreal lake by manipulating N, P, C and reducing light, and measuring PPr, BP, seston C : nutrient ratios and zooplankton biomass in 32 mesocosms. Low light strongly reduced zooplankton biomass in contrast to LNH predictions. PPr did not decrease with low light as predicted by the LNH, however, the phytoplankton community shifted towards low light adapted, but potentially less edible phytoplankton species, such as colony forming Dinobryon (Chrysophyta) and gymnoid (Dinoflagellata) taxa, which were negatively correlated with zooplankton biomass. Seston C : nutrient ratios did not decrease with reduced light, possibly due to the high abundance of mixotrophic phytoplankton across treatments. BP decreased with low light and correlations between BP, bacterial biomass, ciliates and zooplankton suggest that bacteria may be coupled with zooplankton biomass. Thus, the LNH was inadequate when predicting changes in crustacean zooplankton biomass in this typical oligotrophic boreal system, where Daphnia is rare and mixotrophic phytoplankton are abundant. Instead, alternative explanations, such as changes in phytoplankton edibility and energy transferred through the microbial food chain may need investigation to explain reduced zooplankton biomass in low light


INTRODUCTION
Anthropogenic changes to the global nitrogen (N), phosphorus (P) and carbon (C) cycles (Falkowski et al. 2000) influence the amount of light and nutrients available for phytoplankton primary production (PPr) and bacterial production (BP) in the pelagic zone of lakes (Sterner et al. 1997).Changes in these elements may also control the stoichiometry (C : nutrient ratios) of phytoplankton and bacteria, as these elements are essential structural components of phospho-lipids (P and C), amino acids (N and C), carbohydrates (C), nucleic acids (C, N, and P), and light is the primary energy source for phytoplankton.Taken together PPr and BP constitute the total resource base available for crustacean zooplankton and higher trophic levels in the pelagic zone of lakes (Jones 1992).Crustacean zooplankton functions as a central link between fish production and basal production.Zooplankton production tends to be more often limited by food nutritional quality (i.e., in terms of elemental and biochemical composition) than food quantity, except in very oligotrophic lakes (TP , 4 lg L À1 ) (Persson et al. 2007).Therefore, changes in the magnitude of BP and PPr may be less important than changes in the nutritional quality of bacteria and phytoplankton biomass for zooplankton production.
Light availability may not only affect PPr, but also the C : nutrient ratio of phytoplankton, and thus, the quality of phytoplankton as food for higher consumers (Sterner and Elser 2002).In the classic case of the light : nutrient hypothesis (LNH), when light availability is high, but nutrients are limiting, phytoplankton tend to have high C : nutrient ratios, as the excess available C can be stored (Elser et al. 2003).This can create a mismatch between producer supply and grazer demands for nutrients, as zooplankton generally have less variable stoichiometric ratios than phytoplankton (Andersen and Hessen 1991).As a consequence of this, although phytoplankton biomass may be high under high light conditions, the quality of this biomass as a food source for higher trophic levels may be low (Sterner and Elser 2002).According to the LNH, if this mismatch becomes sufficiently strong, at high light conditions crustacean zooplankton production becomes nutrient limited, but in low light conditions production will be C limited (Sterner et al. 1997).
The LNH has proved to be a good predictor of energy transfer in aquatic systems where phytoplankton with highly flexible stoichiometric ratios are abundant (Dickman et al. 2006), and Daphnia is the keystone grazer (Elser and Urabe 1999, Sterner and Elser 2002, Urabe et al. 2002).However, other factors such as differences in phytoplankton stoichiometric flexibility (Katechakis et al. 2005, Hall et al. 2007), phytoplankton biochemical composition (Brett and Mu ¨ller-Navarra 1997), energy transfer through the microbial food web (Sterner et al. 1997), inducible defences and prey edibility (Hessen and van Donk 1993), and the top-down effects of zooplankton grazing (Hall et al. 2007) also play a role in determining the efficiency of energy transfer in the food web, and perhaps the applicability of the LNH across different aquatic ecosystems.
Not only light, but also nutrient availability has the potential to change basal production quantity and quality.BP relies on both autochthonous (phytoplankton exudates) and allochthonous C (from outside the lake) as C sources (Tranvik 1988, Baines andPace 1991).Hence, increased C exudation by phytoplankton at high light may increase BP.Bacteria are often P-limited due to a relatively inflexible low cellular C:P ratio compared to phytoplankton, and require high P concentrations for production (Vadstein 2000).In contrast, PPr tends to be N-limited in areas with low N deposition such as Northern Sweden (Bergstro ¨m and Jansson 2006).Furthermore, the N-and P-requirements of zooplankton tend to be taxon specific, with cladocerans generally being more prone to P-limitation (Daphnia .Bosmina .Holopedium) and copepods to N-limitation (Andersen and Hessen 1991, Schulz andSterner 1999).Thus, copepod biomass may increase with N-additions, whereas cladoceran biomass may increase with P-additions.
The objective of this study is to test the LNH in an oligotrophic clear water system where PPr is N-limited (Bergstro ¨m and Jansson 2006), the phytoplankton community is dominated by mixotrophs, and Daphnia are rare or absent (Faithfull et al. 2011).This was achieved in a mesocosm experiment with manipulations of inorganic N and P, organic C, and light intensity.We hypothesised that (1) Lower light intensity will decrease the quantity of basal energy available for higher trophic levels by reducing basal production and biomass, via a reduction in PPr.(2) Decreases in basal production and biomass may be partially or totally offset by increases in food quality, as seston should have a higher nutritional value (i.e., lower C:N and C:P ratios) in shaded treatments.(3) N and P additions will increase food quantity by stimulating increases in PPr and BP, respectively.(4) N and P additions will also increase food quality, by decreasing seston C:N and C:P ratios.(5) Cladocerans are predicted to be favoured by low C:P seston ratios whereas copepods should be favoured by low C:N seston ratios.

Study site and experimental design
A 3 week mesocosm experiment was conducted in Lake Aborrtja ¨rn 3 (64829 0 N, 19826 0 E), an oligotrophic clear-water lake (total phosphorus (TP): 9.29 6 0.546 lg PÁL À1 , total nitrogen (TN): 0.416 6 0.171 mg NÁL À1 , dissolved organic carbon (DOC) 3.02 6 0.22 mg CÁL À1 , lake mean 6 1SE on the initial sampling date, n ¼ 2).During the study period (1-22 June 2009) epilimnetic lake temperatures were 12-168C.We used a duplicated 4 2 full factorial design with nitrogen (N), phosphorus (P) and glucose (C) addition and reduced light (shading: S), as treatment factors to give 16 different treatments.The mesocosms consisted of clear open plastic bags (depth 2000 mm, diameter 850 mm, volume 1.13 m 3 ), filled with filtered lake water (100 lm mesh from 1 m depth) and attached to wooden rafts in groups of 16.Half of the mesocosms were covered with shade cloth, which covered the top and the sides of the mesocosms down to 1 m, but could be rolled up for sampling.In the shaded mesocosms photosynthetically active radiation at 1 m depth was reduced by 37% from 91.7 6 2.9 to 58.0 6 2.6 lmolÁm À2 Ás À1 (n ¼ 96).This decrease in light climate corresponds to an approximate 7 mg CÁL À1 increase in DOC concentration (Bergstro ¨m et al. 2001).Nutrient addition treatments were assigned randomly to mesocosms on 1 June after initial sampling.Glucose (D-Glucose, AnalaR), P (Na 2 HPO 4 ) and N (NH 4 NO 3 ) were applied 7 times over the experiment, every 3 days, in the concentrations 221 lg CÁL À1 , 7.86 lg NÁL À1 , 0.714 lg PÁL À1 , to give total additions of 1550 lg CÁL À1 , 55 lg NÁL À1 and 5 lg PÁL À1 (C:N:P ¼ 249:18:1 in treatments with CNP additions, all ratios presented are molar) over the experimental period.On 8 June crustacean zooplankton were collected (100 lm mesh size net) from the lake, sorted to remove predatory zooplankton (mainly Bythotrephes sp.) and added at lake densities to each mesocosm.Subsamples for initial zooplankton biomass were preserved with Lugol's iodine.Three distinct conditions were defined in the experiment: Initial sampling (no effect of treatment), 6 June sampling (treatment effects only) and 22 June sampling (treatment þ zooplankton addition effects).

Sampling
Temperature, oxygen and light intensity were measured in the lake and all mesocosms at 1 m depth during the experiment.On each sampling date we used a Ruttner sampler to obtain an integrated sample (5 L) from each mesocosm: directly below the surface, 1 and 1.5 m depth.Water was collected in acid-rinsed plastic containers (1.5 mol/L, HCl).From this sample subsamples were taken for PPr and BP measurements, chemical analyses and bacteria, phytoplankton and ciliate biomass (see below).On each sampling date rotifers were collected from just below the surface, 1 m and 1.5 m and filtered through a 25 lm mesh net.On 22 June crustacean zooplankton were sampled using a 100 lm mesh net (25 cm diameter), drawn vertically from the bottom of the mesocosms to the surface.Samples for phytoplankton, ciliates, rotifers and zooplankton were preserved with Lugol's iodine.Samples for bacterial biomass were preserved with sterile-filtered glutaraldehyde (2% final concentration).

Chemical analyses
For DOC, 10 mL lake water was filtered through 0.22 lm Millipore filters and acidified with 1 mL 2 mol/L HCl.Dissolved inorganic carbon (DIC) samples were collected in air tight vials.DOC and DIC were analyzed using an infrared gas analyzer (HACH IL500).Samples for dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP) were filtered (acid washed Whatman GF/F) and the filtrate was frozen until analysis at the Limnology Department in Uppsala, Sweden, following the procedures described in Bergstro ¨m and Jansson (2000).Edible seston (0.7-30 lm) from 1 L was filtered onto GF/F filters for analysis of particulate organic C (POC) and N (PON) (pre-ignited filters at 5258C, 5 h), and P (POP) (acid-washed 1 M HCl filters).Filters for POC and PON were measured using a Carlo-Erba elemental analyzer and acetanilide as a standard.Filters for POP and TP were digested in a 5% potassium peroxodi-sulfate solution, autoclaved (1218C, 1 h) and analyzed using the molybdate-blue method (Menzel and Corwin 1965).

Bacterial and phytoplankton production and biomass
For BP triplicate 1.2 mL aliquots plus one TCAkilled control were incubated with 8 lL leucine isotope (specific activity 161 Ci mmol/L, Amersham) for 60-90 min in darkness at in situ temperature.The incubation was ended by adding 65 lL 100% TCA and followed the method in Karlsson et al. (2002).Subsamples for bacterial biomass were filtered onto black 0.2 lm, 25 mm diameter polycarbonate filters and stained with acridine orange.Prepared slides were analyzed with epifluorescence microscopy and the image analysis program LabMicrobe (BioRAS, Denmark).Bacterial C biomass was calculated as 0.106 pg CÁlm À3 (Nagata 1986).Samples for PPr were incubated in 120 mL borosilicate glass bottles after adding 1 lL NaH 14 CO 3 (2 mCi/mL) (Amersham, CFA3).For each mesocosm, duplicate light bottles and a single dark bottle were incubated for 4 h over midday at 1 m depth.Bottles were kept dark during transport to the laboratory.Aliquots of 5 mL were placed in glass scintillation vials and acidified with 50 lL 1.5 mol/L HCl, shaken and left open overnight to remove residual inorganic 14 C and carbonates.The following day we added 7.5 mL Optiphase 'Hisafe' 3 multipurpose scintillation cocktail and measured 14 C activity in a Beckman-Coulter LS6500 multi-purpose scintillation counter.Samples for phytoplankton and ciliate identification and biomass were counted using inverted phase-contrast microscopy at 4003 and 1003 magnification after sedimentation of 25 mL for 24 h.Phytoplankton and ciliates were identified to genus level and the biometry of 10 cells from each taxa in each sample were measured.Biovolumes were calculated using biometry measurements and geometrical formulas (Wetzel and Likens 2001) and transformed to biomass (lg/L wet wt.) by assuming a density of 1 gÁcm À3 .Phytoplankton biomasses were converted to C equivalents by assuming a C content of 22% for cyanophytes, 16% for chlorophytes and 11% for other phytoplankton (Blomqvist et al. 1995).Ciliate C biomass was calculated as 0.11 pgÁCÁlm À3 (Turley et al. 1986).Samples for chl-a analysis were filtered directly onto Whatman GF/ F filters, frozen, extracted for 24 h with 95% ethanol in darkness and fluorescence measured with a luminescence spectrometer (Perkin Elmer LS45).Crustacean zooplankton and rotifer samples were photographed, measured and identified to species level where possible with the help of the program Image-Pro plus 4.6.1 (Media Cybernetics).The lengths of all individuals were measured and transformed to dry weight using length-weight regressions (Bottrell et al. 1976).

Statistical analysis
All data were log 10 (x þ 1) transformed prior to analysis to obtain a normal distribution.Factorial analysis of variance (ANOVA) was conducted for each sampling occasion separately as each date represented a distinct combination of treatments (see Methods: Study site and experimental design).There were only a couple of significant interactions between treatment factors in the four-way ANOVAs, therefore, we have presented pooled results for each treatment factor (e.g., N ¼ N, SN, NP, CN, SNP, SCNP, noN ¼ Control, P, C, S, CP, SP, SC, SCP), except where indicated.Change with treatment factor was calculated as: mean treatment factor response (n ¼ 16)/mean response without the treatment factor (n ¼ 16).To determine what caused the zooplankton biomass decline in low light treatments, we used multiple regression models and included as explanatory variables only those parameters that responded significantly to reduced light in the factorial ANOVAs after 6 d (before zooplankton addition) and seston C:P, C:N and N:P ratios.Backward selection was used to find the simplest model based on the Akaike Information Criterion (Crawley 2002).All statistical procedures were carried out in the R program (2.10.0)(R Development Core Team 2009).

Changes in nutrients over time
For TN, TP, DOC, DIC and POC there were no significant differences between treatment factors on the initial sampling (before treatment addition) occasion (factorial ANOVA p , 0.05) (Table 1).However, nutrient concentrations changed in the mesocosms over time and with treatment addition (Table 1).TN was higher with N-addition on 22 June (Table 1) and TP increased with P-addition on both 6 June and 22 June (Table 1).DIC and DOC were not affected by treatment factor (Table 1).However, DIC decreased over the experimental duration (F 1,16 ¼ 9.1, p ¼ 0.004), and DOC accumulated in the mesocosms (F 1,16 ¼ 15.9, p , 0.001).

Food quantity
POC (used as a proxy of food quantity), increased from the initial sampling to 6 June, and was lowest on 22 June, after zooplankton addition (Table 1).POC was significantly higher in the low light mesocosms, but only after zooplankton addition (Table 1).Initial production and biomass measurements did not vary with treatment factor, except for PPr, which was higher in the mesocosms that would become low light mesocosms (Table 2).PPr did not differ with treatment factor on 6 June, but by 22 June PPr had increased with both N-and P-additions (Fig. 1A, Table 2).Phytoplankton biomass was higher in N-addition treatments on 6 June, and by 22 June it had increased in both N-addition and low light treatments, but did not change with Paddition (Fig. 2A, Table 2).Chl-a concentrations were also higher in N-addition and in low light treatments on 22 June (Table 2).Chl-a per unit phytoplankton biomass was significantly higher in low light treatments on 22 June (0.456 0.03) than in ambient light treatments (0.35 6 0.01, F 1,16 ¼ 15.0, p ¼ 0.002).There were no interaction effects of treatment factor on PPr, phytoplankton biomass or chl-a.
PPr:BP ratio did not differ between treatments on 6 June, but, by 22 June PPr:BP had increased with N-addition (Fig. 1C, Table 2).The highest total basal production (PPr þ BP) was recorded with N-additions, although PPr þ BP was higher with both N-and P-additions on 22 June (Table 2).Although PPr þ BP tended to be lower in low light treatments, this difference was not statistically significant (Table 2).Rates of PPr per unit phytoplankton biomass and BP per unit bacterial biomass did not differ among the treatment factors N, P, C and S over the course of the experiment (results not shown).Therefore mea-sured PPr and BP were not dependent on standing biomass stocks.

Food quality
Seston C:N ratio was lower in treatments with N-addition by 22 June, but did not change due to reduced light and ranged from 9.5-15.5:1over the experimental duration (Fig. 1D, Table 2).The C:P ratio of seston was 202:1 6 2.36 across all mesocosms on the initial sampling date and decreased over the experimental duration, showing the sharpest decline after zooplankton addition from 173:1 6 2.47 on 6 June to 132:1 6 Note: Significant P-values ( p , 0.05) are highlighted in bold.Abbreviations are as follows, BP: bacterial production, PPr: phytoplankton primary production, Chl-a: chlorophyll-a, C: carbon, N: nitrogen, P: phosphorus, S: shading.

Zooplankton
Biomasses of all zooplankton taxa were significantly lower in low light mesocosms compared to ambient light treatments by 22 June (Fig. 2B, Table 3).However, the size of the shading effect varied between taxa, with Daphnia showing the greatest percentage decline in reduced light treatments, followed by Holopedium gibberum, Bosmina, cyclopoid copepods, copepod nauplii and calanoid copepods (Table 3).H. gibberum biomass was also somewhat lower in C-addition treatments (21 6 5.7%, F 1,16 ¼ 4.5, p ¼ 0.049).The only interaction effect of the treatment factors was that cyclopoid copepod biomass showed a greater decrease in low light treatments with Paddition than with P-addition alone (Fig. 2B, F 1,16 ¼ 6.4, p ¼ 0.022).Neither P-nor N-additions affected the biomass of any crustacean zooplankton taxa.
Of the variation in zooplankton biomass, 22-54% could be explained by POC, PPr, BP, C:N, C:P, N:P, gymnoid, ciliate or rotifer biomass (Table 4).Biomasses of all zooplankton taxa except calanoid copepods were positively correlated with BP.Gymnoid biomass was negatively correlated with Bosmina, H. gibberum and calanoid copepod biomasses.Biomass of the small ciliate U. agilis was positively correlated with Bosmina, H. gibberum and Daphnia biomasses.With regards to seston stoichiometry, Bosmina was negatively correlated with seston C:P, calanoid copepods were negatively correlated with seston C:N and Daphnia was negatively correlated with seston N:P ratio.This is the opposite pattern we would expect if Bosmina and calanoids were limited by P and N, respectively.Biomass of the rotifer K. longispina was positively correlated with copepod nauplii and H. gibberum biomasses (Table 4).

DISCUSSION
Crustacean zooplankton biomass was lower in reduced light treatments compared to ambient light treatments.Our first hypothesis predicts that zooplankton biomass will decrease with low light due to decreased basal production and biomass.However, PPr and phytoplankton biomass did not decrease with shading as predicted.We also hypothesized that lowered C : nutrient ratios causing an increase in food quality in shaded treatments might offset the decline in zooplankton biomass due to reduced food availability.However, seston C : nutrient ratios were already under zooplankton threshold C : nutrient ratios (see discussion below) before treatment addition, and were unaffected by light.PPr and BP increased with N and P additions, respectively, as predicted by our third hypothesis.Additionally, seston C:N and C:P ratios decreased in N-and P-addition treatments,  v www.esajournals.orgwhich agrees with our fourth hypothesis.However, in contrast to our fifth hypothesis, zooplankton biomass did not increase with nutrient additions, and was unaffected by C : nutrient ratios, which suggests that zooplankton were not nutrient limited in the mesocosms.Consequently, we offer two alternative possibilities to explain the marked reduction in zooplankton biomass in low light treatments: (1) less energy was mobilized through the microbial food web due to lower BP, or (2) a shift in the phytoplankton community composition towards dominance of less edible taxa.These two possibilities, which are not mutually exclusive, are discussed below.
PPr and phytoplankton biomass were unaffected by low light.Additionally, the amount of edible POC did not differ between treatments before zooplankton addition.Previous studies have measured PPr declines at 50% (Llames et al. 2009) and 10% of incident light intensities (Berglund et al. 2007).However, these studies did not report species shifts in phytoplankton community composition, or photo-adaptation (higher chl-a per unit phytoplankton biomass), as found in our study.Here Dinobryon (chrysophyta) and gymnoid phytoplankton taxa became dominant in the low light treatments.Bacterial ingestion by chrysophytes and gymnoids (mixotrophy) (Caron et al. 1993), along with pigmentation adapted to low light (Johnsen and Sakshaug 1996), and the ability to migrate within the water column to reach optimal light intensities (Clegg et al. 2003), may enable these phytoplankton taxa to outcompete obligate autotrophs at low light.Bramm et al. (2009) found a similar phytoplankton community composition shift towards a higher percentage of chrysophytes at ,50% ambient light.Thus, the change in phytoplankton species composition towards a higher abundance of low light adapted taxa may have compensated for the lower light availability in shaded treatments, thus allowing PPr to remain high.
BP was reduced in low light treatments.It is possible that autochthonous-C exuded by phytoplankton was lower in the low light treatments, thus reducing this C-source for BP (Baines and Pace 1991).Alternatively, lower zooplankton biomasses in low light treatments may have reduced community C regeneration rates (Elser and Urabe 1999).However, biomasses of zooplankton taxa (except calanoid copepods and Bosmina) were positively correlated with BP measured on 6 June, before zooplankton addition (Table 4).Although some zooplankton can directly consume bacteria, such as the filter feeders Bosmina, H. gibberum and Daphnia, most zooplankton are inefficient bacterial grazers (Hessen 1985, Vaque ´and Pace 1992).Bacteria are generally considered to be poor food quality, as they lack sterols and essential fatty acids required by zooplankton for production (Brett and Mu ¨ller-Navarra 1997), and Daphnia is unable to survive for extended periods on bacteria alone (Martin-Creuzburg et al. 2011;A. Wenzel, unpublished manuscript).Bacteria may, therefore, to a large extent be transferred to zooplankton through predation on intermediate trophic levels such as bacterial grazing protists, some of which v www.esajournals.orgcan perform a trophic upgrading function, thus, converting poor quality bacteria into high quality zooplankton prey (Bec and Desvilettes 2009).Ciliates and mixotrophs may have provided the link between bacteria and zooplankton, as bacterial biomass was negatively correlated with total ciliates and the biomass of the small ciliate U. agilis.Single celled mixotrophs and U. agilis are both within the edible size range for cladocerans and copepods (Cyr 1998).As zooplankton biomass did not increase in P-addition mesocosms where BP was highest, but decreased with lowered BP in low light treatments, this suggests that zooplankton may require a threshold amount of BP for production.
Zooplankton did not respond to increased nutrient additions or lowered C:P ratios, as predicted in hypothesis four, as seston C:P stoichiometry was always below the threshold elemental ratios 200-400:1 above which growth rates of Daphnia (which is a species with high P requirements) start to decline (Sterner andElser 2002, Anderson andHessen 2005).Although zooplankton N-limitation is more poorly documented than P-limitation in freshwaters, edible seston C:N ratios were similar to reported threshold elemental C:N ratios for Daphnia galeata (11.1:1) and Bosmina longirostris (10:1) at low food abundance (Urabe and Watanabe 1992).Following zooplankton addition seston C:P ratios declined markedly, but not C:N ratios.This suggests that either zooplankton are more efficient P-recyclers, or that they were retaining N, because N availability was lower in the mesocosms.The stability of the seston stoichiometry was probably mediated by bacteria, mixotrophic phytoplankton and flagellates.Pelagic taxa vary in their degrees of stoichiometric flexbility (Hall et al. 2007, Persson et al. 2010), as mixotrophic phytoplankton and bacteria often have less variable stoichiometric ratios than chlorophytes, e.g., Scenedesmus (Vadstein 2000, Katechakis et al. 2005, Chrzanowski et al. 2010).C:P and C:N nutrient ratios of Scenedesmus vary by .10fold, due to storage of large starch granules (DeMott et al. 1998), and Scenedesmus has been used extensively to test the LNH (DeMott et al. 1998, Katechakis et al. 2005).Other phytoplankton taxa maintain relatively constant C : nutrient ratios (i.e., Cryptomonas sp.Rhodomonas sp.), and may do this by exuding excess C at high light levels, or decreasing photosynthetic rates (Panzenbock 2007), especially if nutrients are limiting (Obernosterer and Herndl 1995).In addition, mixotrophs can consume bacteria in low light conditions to overcome energy limitation, but can also consume bacteria in high light conditions to obtain P and, thus, compensate for low dissolved P availability (Caron et al. 1993, Katechakis et al. 2005), hence, keeping seston C:P low.Therefore mixotrophs may have limited the effect of low light on seston stoichiometry.Thus, it appears that the LNH requires a substantial biomass of the phytoplankton community to have stoichiometrically flexible C:N and C:P ratios before edible seston C : nutrient ratios will respond to light manipulation.
Dinobryon spp.and gymnoids made up a higher proportion of the phytoplankton community in low light treatments, and Bosmina, H. gibberum and calanoid copepods were negatively correlated with gymnoid biomass existing on day 6 before zooplankton addition.Therefore high biomasses of Dinobryon spp.and gymnoids may have decreased the edibility of phytoplankton for zooplankton grazers in low light treatments.Dinobryon spp.can form large branching colonies that may function as an induced defense against zooplankton grazing.In addition, some mixotrophs, including chrysophytes, may produce antibiotics or toxic substances at low light and when grazing on bacteria (Katechakis et al. 2005, Blom andPernthaler 2010).In agreement with our results, C. Ja ¨ger (pers.comm.)found that Dinobryon divergens were high quality food for Daphnia galeata at high light levels, but that D. galeata could not survive on D. divergens at low light levels.Some gymnoids can also employ chemical (Kubanek et al. 2007) and behavioral (Selander et al. 2011) defenses against zooplankton grazing.
Variations in ambient light vary with depth and color in boreal lakes, and can be much greater than the 37% reduction in light at 1m depth that we have manipulated (Kalff 2003).Therefore changes in phytoplankton community composition and energy transferred through the microbial food web and their effects on zooplankton biomass may be greater in natural systems than found in our mesocosm experiment.Previous research has shown that production of higher trophic levels increases with annual light availability in lake ecosystems, and has linked this to higher benthic algal production with increased light (Karlsson et al. 2009).However, our study suggests that zooplankton biomass can also be reduced at low light even when grazing only in the pelagic zone.Consequently, processes occurring in the pelagic zone of lakes may also affect the biomass of higher trophic levels depending on light availability.
To conclude, using predictions derived from the LNH we were unable to predict the mechanisms behind zooplankton nutrient and energy limitation of the pelagic food web in a typical oligotrophic boreal lake.Thus, predictions generated by the LNH are inadequate to explain the mechanisms behind changes in crustacean zooplankton biomass due to light and nutrient availability when applied to system where PPr is N-limited, Daphnia is rare or absent and mixotrophic phytoplankton are dominant.Instead, changes in the rate of BP and phytoplankton edibility at lower light intensities appeared to be possible mechanisms controlling zooplankton biomass in this system.

Fig. 2 .
Fig. 2. Biomasses of (A) phytoplankton taxa and bacterial biomass (stacked means 6 1 SE, sum of stacked bars) and (B) crustacean zooplankton taxa (stacked means 6 1 SE, total of all groups combined) on 22 June.All treatment combinations of nitrogen (N), phosphorus (P), carbon (C) and low light (shading: S) are shown (n ¼ 2 for each bar).

Table 1 .
Nutrient grand means (61 SE, n ¼ 32)for all mesocosms are indicated in italics, and differences in nutrients from the grand means (e.g., N ¼ all treatments with N-addition À all treatments without N-addition, 61 SE, n ¼ 16), depending on treatment factor (TF) are shown in regular font.

Table 3 .
Percentage decrease in crustacean zooplankton taxa biomass (means 6 1 SE) in the low light treatment compared to the ambient light treatment.