Cascading Trophic Interactions in an Oligotrophic Species-poor Alpine Lake

Non-native brook trout (Salvelinus fontinalis) were eradicated from alpine Bighorn Lake, Alberta, Canada, to test whether strong cascading trophic interactions (CTI) can occur in oligotrophic, high seston C:P, species-poor lakes. Fishless alpine Pipit Lake was used as a reference ecosystem. Bighorn Lake zooplankton biomass increased from 0.14:1 relative to Pipit Lake before fish removal began in 1997 to 0.6:1 afterwards due to an increase in the abundance of adult cyclopoid copepods beginning in 1997 and the reappearance of Daphnia middendorffiana in 1998. Following the reappearance of Daphnia, Bighorn Lake total phytoplankton biomass fell from 64:1 relative to Pipit Lake to 0.9:1. Over the same periods Bighorn Lake:Pipit Lake chlorophyll-a ratios declined from 2.4:1 to 1.6:1, although the decrease was not statistically significant. Mid-summer Secchi disc depth in Bighorn Lake increased from 3.1 m before manipulation to 9.2 m, the maximum depth of the lake, in 2001 and 2002. Increased transparency was most likely due to increased filtration of suspended inorganic particles from the water column by higher abundances of large zooplankton. Post-manipulation increases in dissolved inorganic nitrogen (DIN), DIN:total dissolved phosphorus (TDP) ratio and declines in TDP in Bighorn Lake were not attributable to ecosystem manipulation, similar changes were observed in reference Pipit Lake. We conclude that strong pelagic CTI, expressed as change in total phytoplankton biomass and largely mediated by Daphnia, can occur in oligotrophic, high seston C:P, species-poor ecosystems. However, strong CTI responses in phytoplankton biomass may lag trophic manipulation by several years.     

Stoichiometric Constraints on Food-Web Dynamics: A Whole-Lake Experiment on the Canadian Shield

A whole-lake manipulation of food-web structure (introduction of a top predator, northern pike, to a minnow-dominated lake) was performed in a Canadian Shield lake (L110) to examine the stoichiometric consequences of changes in planktonic community structure generated by altered food-web structure. Minnow abundance, zooplankton biomass and community composition, microconsumer abundance, and concentration and carbon–phosphorus (C:P) ratio of suspended particulate matter were monitored in L110 and unmanipulated L240 before (1992) and after (1993–95) pike introduction. Algal biomass in L110 determined from microscopic examination for postmanipulation and premanipulation periods was also compared with dynamics in a suite of unmanipulated reference lakes from long-term monitoring records. Pike were added in spring in 1993 and 1994 in sufficient quantity to raise pike biomass to levels of around 22 kg ha^{− 1} by 1994. Minnow populations in L110 responded dramatically, decreasing to levels 30% (1993), 10% (1994), and less than 1% (1995) of premanipulation values. However, most components lower in the food web did not respond in a manner consistent with predictions of existing food-web theory, such as the idea of cascading trophic interactions (CTI). While Daphnia biomass increased in L110 in the first year following manipulation, consistent with CTI, this effect was temporary and Daphnia collapsed in 1995, the year of lowest minnow abundance. Total zooplankton biomass in both lakes declined during the study period and, contrary to CTI, this decline appeared somewhat stronger in L110 than in L240. Dominant microconsumers (heterotrophic microflagellates) did not differ among years in either lake and did not appear to respond to food-web manipulation. At the bottom of the food web, no changes in bacterial biomass occurred in either lake. However, total concentrations of particulate matter appeared to increase in L110 after manipulation (contrary to expectations based on the theory of CTI) while algal biomass did not change in the manipulated lake relative to reference systems. Finally, particulate C:P increased in both L110 and L240 during the study period. The lack of strong response of Daphnia, the lack of response of the microbial food web, decreases in zooplankton biomass and increases in particulate biomass following reduction of minnow populations after piscivore introduction are at odds with expectations from existing food-web theory, such as the idea of CTI as currently formulated. However, the extremely high C:P ratios in particulate matter at the base of the food webs in these lakes, the coincidence of zooplankton declines and increases in particulate C:P ratios, and the results of small-scale mesocosm food-quality experiments are consistent with a hypothesis of a stoichiometric constraint operating on food-web dynamics in this and similar ecosystems. Received 22 April 1997; accepted 8 July 1997.     

Effects of zooplankton on nutrient availability and seston C : N : P stoichiometry in inshore waters of Lake Biwa, Japan

Forty-eight-hour experimental manipulations of zooplankton biomass were performed to examine the potential effects of zooplankton on nutrient availability and phytoplankton biomass (as measured by seston concentration) and C : N : P stoichiometry in eutrophic nearshore waters of Lake Biwa, Japan. Increasing zooplankton, both mixed-species communities and Daphnia alone, consistently reduced seston concentration, indicating that nearshore phytoplankton were generally edible. The zooplankton clearance rates of inshore phytoplankton were similar to rates measured previously for offshore phytoplankton. Increased zooplankton biomass led to increased concentrations of nutrients (NH₄-N, soluble reactive phosphorus [SRP]). Net release rates were higher than those found in previous measurements made offshore, reflecting the nutrient-rich nature of inshore seston. Zooplankton nutrient recycling consistently decreased TIN : SRP ratios (TIN = NH₄ + NO₃ + NO₂). This effect probably resulted from the low N : P ratios of nearshore seston, which were lower than those commonly found in crustacean zooplankton and thus resulted in low retention efficiency of P (relative to N) by the zooplankton. Thus, zooplankton grazing inshore may ameliorate algal blooms due to direct consumption but tends to create nutrient supply conditions with low N : P, potentially favoring cyanobacteria. In comparison with previous findings for offshore, it appears that potential zooplankton effects on phytoplankton and nutrient dynamics differ qualitatively in inshore and offshore regions of Lake Biwa. Received: September 4, 2000 / Accepted: January 23, 2001     

The effects of Daphnia on nutrient stoichiometry and filamentous cyanobacteria: a mesocosm experiment in a eutrophic lake

1. Stoichiometric theory predicts that the nitrogen : phosphorus (N : P) ratio of recycled nutrients should increase when P‐rich zooplankton such as Daphnia become dominant. We used an enclosure study to test the hypothesis that an increased biomass of Daphnia will increase the relative availability of N versus P sufficiently to decrease the abundance of filamentous cyanobacteria. The experiment was conducted in artificially enriched Lake 227 (L227) in the Experimental Lakes Area (ELA), north‐western Ontario, Canada. Previous studies in L227 have shown that the dominance of filamentous, N‐fixing cyanobacteria is strongly affected by changes in the relative loading rates of N and P. 2. We used a 2 × 2 factorial design with the addition or absence of D. pulicaria and high or low relative loading rates of N and P (+NH₄, –NH₄) in small enclosures as treatment variables. If Daphnia can strongly affect filamentous cyanobacteria by altering N and P availability, these impacts should be greatest with low external N : P loading rates. The phytoplankton community of L227 was predominantly composed of filamentous Aphanizomenon spp. at the start of the experiment. 3. Daphnia strongly reduced filamentous cyanobacterial density in all enclosures to which they were added. The addition of NH₄ had only a small impact on algal community composition. Hence, we conclude that Daphnia did not cause reductions in cyanobacteria by altering the N : P ratio of available nutrients. 4. Despite the lack of evidence that Daphnia affected filamentous cyanobacteria by altering the relative availability of N and P, we found changes in nutrient cycling consistent with other aspects of stoichiometric theory. In the presence of Daphnia, total P in the water column decreased because of an increase in P sedimentation. In contrast to P, a decrease in suspended particulate N was offset by an increase in dissolved N (especially NH₄). Hence, dissolved and total N : P ratios in the water column increased with Daphnia as a result of differences in the fate of suspended particulate N versus P. There was minimal accumulation and storage of P in Daphnia biomass in the enclosures. 5. Our experiment demonstrated that Daphnia can strongly limit filamentous cyanobacterial abundance and affect the biogeochemical cycling of nutrients. In our study, changes in nutrient cycling were apparently insufficient to cause the changes in phytoplankton community composition that we observed. Daphnia therefore limited filamentous cyanobacteria by other mechanisms.     

Inter- and intra-annual variability in the phytoplankton community of a high mountain lake: the influence of external (atmospheric) and internal (recycled) sources of phosphorus

1. The inter‐ and intra‐annual changes in the biomass, elemental (carbon (C), nitrogen (N) and phosphorus (P)) and taxonomical composition of the phytoplankton in a high mountain lake in Spain were studied during 3 years with different physical (fluctuating hydrological regime) and chemical conditions. The importance of internal and external sources of P to the phytoplankton was estimated as the amount of P supplied via zooplankton recycling (internal) or through ice‐melting and atmospheric deposition (external). 2. Inter‐annual differences in phytoplankton biomass were associated with temperature and total dissolved phosphorus. In 1995, phytoplankton biomass was positively correlated with total dissolved phosphorus. In contrast, the negative relationship between zooplankton and seston biomass (direct predatory effects) and the positive relationship between zooplankton P excretion and phytoplankton biomass in 1997 (indirect P‐recycling effects), reinforces the primary role of zooplankton in regulating the total biomass of phytoplankton but, at the same time, encouraging its growth via P‐recycling. 3. Year‐to‐year variations in seston C : P and N : P ratios exceeded intra‐annual variations. The C : P and N : P ratios were high in 1995, indicating strong P limitation. In contrast, in 1996 and 1997, these ratios were low during ice‐out (C : P < 100 and N : P < 10) and increased markedly as the season progressed. Atmospheric P load to the lake was responsible for the decline in C : P and N : P ratios. 4. Intra‐annual variations in zooplankton stoichiometry were more pronounced than the overall differences between 1995 and 1996. Thus, the zooplankton N : P ratio ranged from 6.9 to 40.1 (mean 21.4) in 1995, and from 10.4 to 42.2 (mean 24.9) in 1996. The zooplankton N : P ratio tended to be low after ice‐out, when the zooplankton community was dominated by copepod nauplii, and high towards mid‐ and late‐season, when these were replaced by copepodites and adults. 5. In 1995, the minimum demands for P of phytoplankton were satisfied by ice‐melting, atmospheric loading and zooplankton recycling over 100%. In order of importance, atmospheric inputs (> 1000%), zooplankton recycling (9–542%), and ice‐melting processes (0.37–5.16%) satisfied the minimum demand for P of phytoplankton during 1996 and 1997. Although the effect of external forces was rather sporadic and unpredictable in comparison with biologically driven recycle processes, both may affect phytoplankton structure and elemental composition. 6. We identified three conceptual models representing the seasonal phosphorus flux among the major compartments of the pelagic zone. While ice‐melting processes dominated the nutrient flow at the thaw, biologically driven processes such as zooplankton recycling became relevant as the season and zooplankton ontogeny progressed. The stochastic nature of P inputs associated with atmospheric events can promote rapid transitional changes between a community limited by internal recycling and one regulated by external load. 7. The elemental composition of the zooplankton explains changes in phytoplankton taxonomic and elemental composition. The elemental negative balance (seston N : P < zooplankton N : P, low N : P recycled) during the thaw, would promote a community dominated by species with a high demand for P (Cryptophyceae). The shift to an elemental positive balance (seston N : P > zooplankton N : P, high N : P recycled) in mid‐season would skew the N : P ratio of the recycled nutrients, favouring dominance by chrysophytes. The return to negative balance, as a consequence of the ontogenetic increase in zooplankton N : P ratio and the external P inputs towards the end of the ice‐free season, could alleviate the limitation of P and account for the appearance of other phytoplankton classes (Chlorophyceae or Dinophyceae).     

Interactive effects of higher temperature and dissolved organic carbon on planktonic communities in fishless mountain lakes

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Field evidence for stoichiometric relationships between zooplankton and N and P availability in a shallow calcareous lake

1. According to stoichiometric theory, zooplankters have a species‐specific elemental composition. Daphniids have a relatively high phosphorus concentration in their tissues and copepods high nitrogen. Daphniids should, therefore, be more sensitive to phosphorus limitation and copepods more sensitive to nitrogen. A 2‐year study of a shallow marl lake in the west of Ireland investigated whether population fluctuations of the two dominant taxa, Daphnia spp. and the calanoid Eudiaptomus gracilis, were associated with the availability of phosphorus and nitrogen. 2. In accordance with stoichiometric predictions, Daphnia and Eudiaptomus reproduction had contrasting relationships with dietary phosphorus and nitrogen availability. Egg production by Daphnia was negatively associated with the ratio of dissolved inorganic nitrogen (DIN) : total phosphorus (TP) and the ratio of light to TP which was used as an indirect index for seston carbon (C) : phosphorus (P). Conversely calanoid egg production had a positive relationship with the DIN : TP ratio and was unrelated to the estimated C : P (light : TP) ratio. 3. Daphnia biomass was not, however, correlated with phosphorus availability, and neither was calanoid biomass correlated with nitrogen. The high ratio of DIN : TP when Daphnia dominated the zooplankton biomass and the low ratio when calanoids dominated, is consistent with Daphnia acting as a sink for phosphorus and calanoids as a sink for nitrogen and suggests consumer‐driven nutrient recycling.     

Restoration by biomanipulation in a small hypertrophic lake: first-year results

Biomanipulation was carried out in order to improve the water quality of the small hypertrophic Lake Zwemlust (1.5 ha; mean depth 1.5 m). In March 1987 the lake was drained to facilitate the elimination of fish. Fish populations were dominated by planktivorous and benthivorous species (total stock c. 1500 kg) and were collected by seine- and electro-fishing. The lake was subsequently re-stocked with 1500 northern pike fingerlings (Esox lucius L.) and a low density of adult rudd (Scardinius erythrophthalmus). The offspring of the rudd served as food for the predator pike. Stacks of Salix twigs, roots of Nuphar lutea and plantlets of Chara globularis were brought in as refuge and spawning grounds for the pike, as well as shelter for the zooplankton.The impact of this biomanipulation on the light penetration, phytoplankton density, macrophytes, zooplankton and fish communities and on nutrient concentrations was monitored from March 1987 onwards. This paper presents the results in the first year after biomanipulation.The abundance of phytoplankton in the first summer (1987) after this biomanipulation was very low, and consequently accompanied by increase of Secchi-disc transparency and drastic decline of chlorophyll a concentration.The submerged vegetation remained scarce, with only 5 % of the bottom covered by macrophytes at the end of the season.Zooplankters became more abundant and there was a shift from rotifers to cladocerans, comprised mainly of Daphnia and Bosmina species, the former including at least 3 species.The offspring of the stocked rudd was present in the lake from the end of August 1987. Only 19% of the stocked pike survived the first year.Bioassays and experiments with zooplankton community grazing showed that the grazing pressure imposed by the zooplankton community was able to keep chlorophyll a concentrations and algal abundance to low levels, even in the presence of very high concentrations of inorganic N and P. The total nutrient level increased after biomanipulation, probably due to increased release from the sediment by bioturbation, the biomass of chironomids being high.At the end of 1987 Lake Zwemlust was still in an unstable stage. A new fish population dominated by piscivores, intended to control the planktivorous and benthivorous fish, and the submerged macrophytes did not yet stabilize.     

Dynamics of a Boreal Lake Ecosystem during a Long-Term Manipulation of Top Predators

We assessed the long-term (16 years) effects of introducing piscivores (northern pike) into a small, boreal lake (Lake 221, Experimental Lakes Area) containing abundant populations of two planktivorous fish species. After the introduction, pearl dace were extirpated and yellow perch abundance was greatly reduced. Daphnia species shifted from D. galeata mendota to larger bodied Daphnia catawba, but the total zooplankton biomass did not increase, nor did the biomass of large grazers such as Daphnia. Phytoplankton biomass decreased after the northern pike introduction, but increased when northern pike were partially removed from the lake. Phosphorus (P) excretion by fish was ∼0.18 mg P m⁻² d⁻¹ before pike addition, declined rapidly to approximately 0.03–0.10 as planktivorous perch and dace populations were reduced by pike, and increased back to premanipulation levels after the pike were partially removed and the perch population recovered. When perch were abundant, P excretion by fish supported about 30% of the P demand by primary producers, decreasing to 6–14% when pike were abundant. Changes in phytoplankton abundance in Lake 221 appear to be driven by changes in P cycling by yellow perch, whose abundance was controlled by the addition and removal of pike. These results confirm the role of nutrient cycling in mediating trophic cascades and are consistent with previous enclosure experiments conducted in the same lake.     

Does excess carbon affect respiration of the rotifer Brachionus calyciflorus Pallas?

1. Herbivorous zooplankton maintain a rather constant elemental composition in their body mass as compared with the variability commonly encountered in their food. Furthermore, their high phosphorus (P) and nitrogen (N) content means that they often face an excess of carbon (C) in their diet. Regulation of this surplus of energy may occur via modulation of assimilation efficiency, or postassimilation by increased respiration (CO₂) and/or excretion dissolved organic carbon, DOC. Whereas several studies have examined the effect of elemental imbalance in the genus Daphnia, few have examined other zooplankton taxa. 2. We investigated whether the rotifer Brachionus calyciflorus uses increased respiration as a means of stoichiometrically regulating excess dietary C. Growth rate and respiration were measured under different food qualities (C : N and C : P ratios). 3. Both C : N and C : P ratios in food had strong effects on growth rate, demonstrating strong nutrient limitation of rotifer growth when nutrient elements were depleted in the diet and indicating the need for stoichiometric regulation of excess ingested C. 4. Respiration measurements, supported by a stoichiometric model, indicated that excess C was not released as CO₂ in B. calyciflorus and that nutrient balance must therefore be maintained by other means such as excretion of DOC or egestion in faecal material.     

Daphnia population growth but not moulting is a substantial phosphorus drain for phytoplankton

SUMMARY 1. Negative effects of zooplankton on the availability of phosphorus (P) for phytoplankton as a result of the retention of nutrients in zooplankton biomass and the sedimentation of exoskeletal remains after moulting, have been recently proposed. 2. In a mesocosm study, the relative importance of these mechanisms was tested for the freshwater cladoceran Daphnia hyalina×galeata. A total of 13 mesocosm bags was suspended in a mesotrophic German lake during summer 2000 and fertilised with inorganic P in order to obtain a total nitrogen to total P ratio closer to the Redfield ratio. D. hyalina×galeata was then added at a logarithmically scaled density gradient of up to 40 ind. L^?1. Zooplankton densities, dissolved inorganic, particulate organic (seston <100 μm), as well as total nutrient concentrations were monitored. Additionally, nutrient concentrations of sediment water removed from the bottom of the mesocosm bags via a manual pump were determined. 3. Seston carbon (C), seston P and total P were significantly negatively correlated with Daphnia densities. The amount of particulate P (～5–6 μg P L^?1) sequestered from the seston compartment by Daphnia corresponded roughly to the increase of zooplankton biomass (population growth). Soluble reactive phosphorous (SRP) was at all times high (～25–35 μg P L^?1) and possibly unavailable to phytoplankton as a result of P adsorption to calcite during a calcite precipitation event (whiting). P concentrations determined in sediment water were generally <60 μg P m^?2 and thus never exceeded 1% of the total amount of P bound in particulate matter of the overlying water column. 4. Seston C : P ratios followed a polynomial second‐order function: At Daphnia densities <40 ind. L^?1 a positive linear relationship was evident, which is explained by the stronger reduction of P compared with C in seston, and transfer of seston P to zooplankton. Highest seston C : P ratios of ～300 : 1 were observed at Daphnia densities of ～30–50 ind. L^?1, which is in agreement with proposed threshold values limiting Daphnia reproductive growth. At Daphnia densities >40–50 ind. L^?1 C : P ratios were decreased because of the strong reduction of seston C at close to constantly low seston P‐values of ～3–4 μg P L^?1. 5. At least for Daphnia, it may be concluded that – unlike population growth – the sedimentation of faecal pellets and carapaces after moulting seem negligible processes in pelagic phosphorus dynamics.     

Atmospheric nitrogen deposition is associated with elevated phosphorus limitation of lake zooplankton

Here, we present data that for the first time suggests that the effects of atmospheric nitrogen (N) deposition on nutrient limitation extend into the food web. We used a novel and sensitive assay for an enzyme that is over‐expressed in animals growing under dietary phosphorus (P) deficiency (alkaline phosphatase activity, APA) to assess the nutritional status of major crustacean zooplankton taxa in lakes across a gradient of atmospheric N deposition in Norway. Lakes receiving high N deposition had suspended organic matter (seston) with significantly elevated carbon:P and N:P ratios, indicative of amplified phytoplankton P limitation. This P limitation appeared to be transferred up the food chain, as the cosmopolitan seston‐feeding zooplankton taxa Daphnia and Holopedium had significantly increased APA. These results indicate that N deposition can impair the efficiency of trophic interactions by accentuating stoichiometric food quality constraints in lake food webs.     

Beneficial and detrimental interactive effects of dissolved organic matter and ultraviolet radiation on Zooplankton in a transparent lake

While changes in dissolved organic matter (DOM) concentrations are expected to affect zooplankton species through attenuation of potentially damaging ultraviolet (UV) radiation, generation of potentially beneficial or harmful photoproducts, pH alteration, and microbial food web stimulation, the combined effects of such changes on zooplankton community structure have not been studied previously. Our purpose was to determine how an increase in allochthonous DOM and associated changes in pH in an initially transparent lake may affect zooplankton community structure, and how exposure to solar UV may alter these DOM and pH effects. We ran microcosm experiments manipulating UV, DOM, and pH near the surface of Lake Giles in northeastern Pennsylvania. We found that when DOM was added in the presence of ambient UV, Daphnia and copepod UV-mortality was reduced by approximately three and two times compared to UV exposure without extra DOM. When DOM was added in the absence of UV, adult Daphnia and copepods were reduced compared to no DOM addition in the absence of UV. Daphnia and cyclopoid egg production and rotifer abundance were generally higher in the presence of DOM, regardless of UV treatment. The lower abundance yet high egg production in the presence of DOM and absence of UV may be explained by higher abundance of egg-bearing adults compared to non-egg-bearers. We conclude that allochthonous DOM benefits some zooplankton in a high-UV environment, but may be detrimental under low-UV conditions. Overall, Daphnia abundance and egg production were higher than that of calanoid copepods in the DOM additions, indicating that in some lakes an increase in allochthonous DOM may lead to a zooplankton community shift favoring Daphnia over calanoid copepods.     

Effect of larval fish and nutrient enrichment on plankton dynamics in experimental ponds

The hypotheses that larval fish density may potentially affect phytoplankton abundance through regulating zooplankton community structure, and that fish effect may also depend on nutrient levels were tested experimentally in ponds with three densities of larval walleye, Stizostedion vitreum (0, 25, and 50 fish m^–3), and two fertilizer types (inorganic vs organic fertilizer). A significant negative relationship between larval fish density and large zooplankton abundance was observed despite fertilizer types. Larval walleye significantly reduced the abundances of Daphnia, Bosmina, and Diaptomus but enhanced the abundance of various rotifer species (Brachionus, Polyarthra, and Keratella). When fish predation was excluded, Daphnia became dominant, but Daphnia grazing did not significantly suppress blue-green algae. Clearly, larval fish can be an important regulator for zooplankton community. Algal composition and abundance were affected more by fertilizer type than by fish density. Inorganic fertilizer with a high N:P ratio (20:1) enhanced blue-green algal blooms, while organic fertilizer with a lower N:P ratio (10:1) suppressed the abundance of blue-green algae. This result may be attributed to the high density of blue-green algae at the beginning of the experiment and the fertilizer type. Our data suggest that continuous release of nutrients from suspended organic fertilizer at a low rate may discourage the development of blue-green algae. Nutrient inputs at a low N:P ratio do not necessarily result in the dominance of blue-green algae.     

Effects of zooplankton dynamics on epilimnetic phosphorus loss

1. Our aim was to analyse the impact of zooplankton dynamics on the relative importance of two mechanisms contributing to the loss of phosphorus (P) from the epilimnion of stratified lakes: net population incorporation into zooplankton biomass and P sedimentation. 2. We established enclosures without Daphnia (control), with a growing Daphnia population (treatment D) and with a high, stable Daphnia population (treatment D+). The P incorporated in zooplankton biomass and sedimented was measured at short intervals over a period of 17 days. 3. In both Daphnia treatments, sedimentation increased and the P content of sedimented matter was higher than in the control and highest in D+. The P loss by sedimentation between day 3 and 17 was generally high (>25%, >1.8% day^?1) with particularly high values in D+ (～60%, 4.3% day^?1). Phosphorus sedimentation was higher in the zooplankton treatments, although the contribution of exuviae and dead Daphnia was minor. Faecal material was probably a major component of sedimentation. 4. By contrast, the amount of P in zooplankton (mainly Daphnia) biomass increased in D but remained constant in D+. Phosphorus loss owing to net population incorporation was generally low and ranged up to 6.5% (0.5% day^?1) in treatment D. A positive relationship between Daphnia dry mass and P sedimentation, as well as P incorporation, was found. 5. Sedimentation is evidently an important cause of P loss from the epilimnion where Daphnia is abundant. By contrast, P incorporation into Daphnia biomass may only become an important loss factor when the population is growing.     

Relative importance of inefficient feeding and consumer excretion to organic carbon flux from Daphnia

1. Sloppy and inefficient feeding by zooplankton is generally thought to make a major contribution to the regeneration of the dissolved organic carbon (DOC) pool in aquatic environments. In this study, we tested experimentally the regeneration of DOC by a freshwater zooplankter feeding on two species of phytoplankton at different food concentrations and C : P ratios. We separated the DOC production because of inefficient feeding (pre‐ingestive regeneration) and zooplankton excretion and faeces release (postdigestive regeneration). 2. Within a brief incubation period (10 min), DOC production in the presence of Daphnia was not significantly different from that in the control treatment without grazers. During a longer incubation period (4 h), the amounts of radiocarbon retained in the algal cells per se were constant or were not different from those in the control treatments. These experimental results strongly suggest that inefficient feeding did not contribute significantly to DOC production in the grazer–prey system. 3. During the 4‐h incubation, calculations of the DOC per ingestion rate (i.e. DOC produced by Daphnia alone) showed that food concentration and algal species did not affect the relative DOC production, but there was considerable difference at different algal C : P ratios and grazer densities. We found that direct excretion of DOC by Daphnia occurred rapidly following food digestion and accounted for >65% of the total DOC production. Maximum DOC leakage from Daphnia faeces contributed less to DOC production than the grazer excretion, except under P‐limited conditions. 4. This study highlights the dominant role of postingestive process, especially the direct excretion by zooplankton, in DOC production in a grazer–prey system.     

Response of phytoplankton and bacteria to nutrients and zooplankton: a mesocosm experiment

Although both nutrient inputs and zooplankton grazing are importantto phytoplankton and bacteria in lakes, controversy surroundsthe relative importance of grazing pressure for these two groupsof organisms. For phytoplankton, the controversy revolves aroundwhether zooplankton grazers, especially large cladocerans likeDaphnia, can effectively reduce phytoplankton populations regardlessof nutrient conditions. For bacteria, little is known aboutthe balance between possible direct and indirect effects ofboth nutrients and zooplankton grazing. However, there is evidencethat bacteria may affect phytoplankton responses to nutrientsor zooplankton grazing through direct or apparent competition.We performed a mesocosm experiment to evaluate the relativeimportance of the effects of nutrients and zooplankton grazingfor phytoplankton and bacteria, and to determine whether bacteriamediate phytoplankton responses to these factors. The factorialdesign crossed two zooplankton treatments (unsieved and sieved)with four nutrient treatments (0, 0.5, 1.0 and 2.0 µgphosphorus (P) l^–1 day^–1 together with nitrogen(N) at a N:P ratio of 20:1 by weight). Weekly sieving with 300µm mesh reduced the average size of crustacean zooplanktonin the mesocosms, decreased the numbers and biomass of Daphnia,and increased the biomass of adult copepods. Nutrient enrichmentcaused significant increases in phytoplankton chlorophyll a(4–5x), bacterial abundance and production (1.3x and 1.6x,respectively), Daphnia (3x) and total zooplankton biomass (2x).Although both total phytoplankton chlorophyll a and chlorophylla in the <35 µm size fraction were significantly lowerin unsieved mesocosms than in sieved mesocosms, sieving hadno significant effect on bacterial abundance or production.There was no statistical interaction between nutrient and zooplanktontreatments for total phytoplankton biomass or bacterial abundance,although there were marginally significant interactions forphytoplankton biomass <35 µm and bacterial production.Our results do not support the hypothesis that large cladoceransbecome less effective grazers with enrichment; rather, the differencebetween phytoplankton biomass in sieved versus unsieved zooplanktontreatments increased across the gradient of nutrient additions.Furthermore, there was no evidence that bacteria buffered phytoplanktonresponses to enrichment by either sequestering P or affectingthe growth of zooplankton.     

The relations between 0+ fish density,zooplankton size and the vulnerability of pikeperch,Stizostedion lucioperca,to angling in the Frisian lakes

During a five year study, the size and abundance of zooplankton and 0+ fish in September were registered in a number of Frisian lakes. The mean size of Daphnia was related to the abundance of planktivorous 0+ fish in September. Immigration of fish larvae into the Frisian Lake District from the IJsselmeer in June was observed in 1979 and 1980. The abundance of 0+ fish was negatively related and the size of Daphnia was positively related to the vulnerability to angling of predatory pikeperch.     

The stoichiometry of N and P in the pelagic zone of Castle Lake, California

We measured the concentrations, as well as lake-wide amounts,of nitrogen (N) and phosphorus (P) in dissolved, seston andzooplankton pools throughout the water column of Castle Lake,California, during summer, 1991. This allowed us to determinethe stoichiometric ratios of important elements in each pool(C:N, C:P, N:P) as well as for the entire lake. Dissolved andseston pools were the predominant storage compartments for bothN and P; zooplankton never contained >5% of N or 10% of Plake wide. However, by late summer, the concentrations of Pin seston and in zooplankton were similar in the upper portionsof the water column, suggesting that changes in food web structurethat alter zooplankton biomass and community composition (andhence elemental storage in the zooplankton) may produce significantshifts in nutrient storage among pelagic pools. Lake-wide levelsof dissolved N were largely constant over the study period;however, lake-wide dissolved P increased. These dynamics suggestedthat the majority of nutrients stored in dissolved pools wereunavailable for phytoplankton growth. N:P and C:P ratios indicatedthat Castle Lake phytoplankton became severely deficient inP during the course of our observations. These ratios also greatlyexceeded recently reported threshold values for elemental constraintson growth and reproduction for several species of zooplankton.The ratio of N to P in the zooplankton pool was relatively constantand consistently lower than that in the sestion. As a result,the predicted N:P ratio of zooplankton-regenerated nutrientsexceeded the N:P ratio of the seston, implying that zooplanktonnutrient regeneration further skewed N and P supply ratios,and potentially enhanced P limitation of phytoplankton in CastleLake. ¹Present address: Department of Biology, Box 19498, Universityof Texas at Arlington, Arlington, TX 76019, USA     

Empirical models for predicting the excretion of nutrients (N and P) by aquatic metazoans: taxonomic differences in rates and element ratios

1. We developed empirical models for predicting the release of nutrients [nitrogen (N) and phosphorus (P)] by aquatic metazoans (zooplankton, mussels, benthic macroinvertebrates and fish). 2. The number of species represented in each model ranged from 9 to 74 (n = 40 – 1122), organism dry mass from 1 × 10^?5 to 8 × 10⁴ mg and water temperature from ?1.8 to 32 °C for all models. Organisms were from marine and freshwater (both lotic and lentic) environments. 3. Rates and ratios of nutrient excretion were modelled and intra‐ and intertaxon differences in excretion were examined. Rates of N and P excretion were not significantly different between marine and freshwater species within the same taxon (e.g. zooplankton). However, rates of excretion (as a function of organism dry mass and water temperature) were significantly different among different orders of zooplankton, mussels and fish. However, excretion of N was similar among different orders of benthic macroinvertebrates. 4. Detritivorous fish excreted both N and P at rates greater than all other taxa; whereas mussels excreted N and P generally at rates less than other taxa. There were no significant differences in the rate of N and P excretion between zooplankton and fish (i.e. the allometry of N and P excretion was similar between zooplankton and fish). 5. Molar N : P ratios of nutrients excreted increased with increasing organism dry mass for each group of metazoans, except for zooplankton and detritivorous fish (where N : P ratios declined with increasing organism dry mass). Molar N : P ratios in the excretions of aquatic metazoans were generally below the Redfield ratio of 16:1. 6. We examined the influence of variable abundance of zooplankton, benthic macroinvertebrates and fish on assemblage excretion rates. Rates of N and P excretion were calculated by applying our models to metazoan biomass and abundance data over seven consecutive years in two oligotrophic lakes. Rates of N and P excretion (g ha^?1 day^?1) increased linearly with increasing assemblage biomass (kg ha^?1). However, rates of N and P excretion were significantly and negatively correlated with the relative abundance of fish and positively correlated with the relative abundance of zooplankton.