Biomanipulation of lake ecosystems: an introduction

SUMMARY 1. This paper is an introduction to a special issue of Freshwater Biology containing selected papers from an international symposium on Food Web Effects of Fish in Lake Ecosystems: Research Progress, Water Quality and Fisheries Management held from 31 May to 3 June 2000 in Rheinsberg, Germany. The primary goal of the workshop was to enlarge the current view of fish-induced effects on lake ecosystems. An additional goal was to promote biomanipulation as a multiple-use tool for managing freshwater ecosystems.
2. The three main topics addressed at the workshop were: (i) mechanisms involved in biomanipulation, (ii) whole-lake case studies and (iii) management aspects in water quality and fisheries.
3. Mortality of Daphnia , nutrient recycling, habitat selection and fish predation are reported as important mechanisms governing food-web effects as a result of biomanipulation.
4. Whole-lake case studies indicate that repeated fish removal can help improve water quality of shallow lakes, but successful biomanipulation of deep, thermally stratifying lakes remains difficult.
5. In many cases, biomanipulation of lakes has proved to provide benefits in addition to improving water quality. As all lake users are potentially affected when biomanipulation is used as a lake management tool, their concerns need to be clearly recognised if biomanipulation is to be successful in practice.     

Piscivore enhancement effects on food webs depend on planktivore body size and species composition in replicated whole lake experiments

Recent syntheses of trophic cascade and biomanipulation research have suggested that the effects of piscivores on planktivorous fish populations are reduced, when planktivores are capable of outgrowing predator gape limitation and in systems with complex food web interactions. These hypotheses, however, have not been tested in long-term, whole-lake, experiments where processes such as fish recruitment and compensatory food web responses may be important. We conducted a replicated whole-lake experiment to test for the effects of supplemental piscivore introductions on food webs of eutrophic lakes dominated by deep-bodied planktivores. Responses to piscivore enhancement were compared between lakes differing in food web structure due to the presence of omnivorous gizzard shad (Dorosoma cepedianum). A significant decrease in the relative abundance of juvenile planktivorous fish, and an increase in total benthic macroinvertebrate density was observed in lakes containing mainly bluegills (Lepomus machrochirus). In contrast, lakes containing gizzard shad exhibited no significant responses to piscivore manipulation. Our results support the hypothesis that food webs in lakes dominated by deep-bodied planktivorous fish species respond weakly to piscivore enhancement. In addition, our findings support the hypothesis that cascading trophic interactions are weaker in lake ecosystems with more complex food web interactions such as those containing gizzard shad.     

Evaluating Alternative Explanations in Ecosystem Experiments

Unreplicated ecosystem experiments can be analyzed by diverse statistical methods. Most of these methods focus on the null hypothesis that there is no response of a given ecosystem to a manipulation. We suggest that it is often more productive to compare diverse alternative explanations (models) for the observations. An example is presented using whole-lake experiments. When a single experimental lake was examined, we could not detect effects of phosphorus (P) input rate, dissolved organic carbon (DOC), and grazing on chlorophyll. When three experimental lakes with contrasting DOC and food webs were subjected to the same schedule of P input manipulations, all three impacts and their interactions were measurable. Focus on multiple alternatives has important implications for design of ecosystem experiments. If a limited number of experimental ecosystems are available, it may be more informative to manipulate each ecosystem differently to test alternatives, rather than attempt to replicate the experiment.     

Interactions between invasive Eurasian watermilfoil and native water stargrass in Cayuga Lake,NY, USA

Aims Eurasian watermilfoil (Myriophyllum spicatum L.) is a common invasive plant in American lakes and has many negative impacts on invaded ecosystems. Drastic decline of this plant at the northern end of Cayuga Lake in the New York State has occurred since the 1980s, with a much smaller magnitude of decline of Eurasian watermilfoil at the southern end (Johnson et al. 2000) During the same period, increases in the abundance of native plants, particularly water stargrass (Heteranthera dubia (Jacq.) MacMill.), have been observed (Johnson et al. (1998)) We aimed to investigate the mechanisms responsible for the decline of Eurasian watermilfoil and evaluate the responses of co-occurring plants at the two ends of Cayuga Lake over time. We hypothesized that plant interactions might have contributed to the drastic decline of Eurasian watermilfoil, particularly allelopathy by native water stargrass.Methods A lake survey was conducted to assess distribution and abundance of plant communities at the northern end and the southern end of Cayuga Lake. Additionally, two sets of greenhouse experiments were conducted to investigate the interactions between invasive Eurasian watermilfoil and native water stargrass. A competition experiment evaluated intra- versus inter-specific competition among plants grown together; an allelopathy experiment examined the responses of plants to each other's extracts.Important findings The lake survey showed that water stargrass was extremely abundant at the northern end, whereas Eurasian watermilfoil was sparse at the northern end but one of the most common species at the southern end. The survey also revealed that water stargrass was more abundant than Eurasian watermilfoil at sites where the two species coexisted in the lake. Results from greenhouse experiments revealed no effects of Eurasian watermilfoil on water stargrass growth. However, Eurasian watermilfoil biomass was reduced by 46% when treated with high concentration of water stargrass extracts. This is likely due to osmotic effects rather than allelopathic effects of water stargrass. We proposed several possible reasons for the drastic decline of Eurasian watermilfoil and the increase in water stargrass abundance at the northern end of Cayuga Lake, including space competition, nutrients, substrates, wind exposure and water clarity in addition to insect herbivory and mechanic harvesting.     

Preliminary Observations on the Nature,Taxonomic Composition,Distribution, Behaviour and Nutrient Requirements of the “Unusual” Algal “Jellies” of Oguta Lake in Southeastern Nigeria

Preliminary observations were made between October 1982 and May 1983 on the nature, taxonomic composition, spatial distribution, buoyancy behaviour and nutrient requirements of some unusual planktonic algal “jellies” in Oguta Lake in Southeastern Nigeria. These “jellies” are macroscopic, blue-green algal masses whose excessive mucilage of high consistency acts as substrata for other epiphytic components viz. diatoms and green algae. They are more abundant in the lentic areas of the lake, undergo diurnal vertical movements and have their growth enhanced by spikes of nitrogen and phosphorus salts into their aquatic medium. The effects of these “jellies” on the ecology and general water use of the lake are discussed. Speculations on the reasons for their occurrence are given and possible control measures outlined.     

Cladocera paleocommunity to disentangle the impact of anthropogenic and climatic stressors on a deep subalpine lake ecosystem (Lake Iseo,Italy)

In big lakes with strong anthropogenic pressure, it is usually difficult to disentangle the impacts of climate variability from those driven by eutrophication. The present work aimed at the reconstruction of change in the species distribution and density of subfossil Cladocera in Lake Iseo (Italy) in relation to climate and anthropogenic pressure. We related subfossil Cladocera species composition and density in an 80-cm sediment core collected in the pelagic zone of Lake Iseo to long-term temperature trends and phosphorus concentration inferred by diatoms frustules. The Cladocera remains detected in Lake Iseo sediment reflected the species composition and density of modern pelagic Cladocera assemblages. Cladocera rapidly respond to environmental change, and that climate change combined with eutrophication can induce changes in community composition and species density. At the beginning of twentieth century, when global warming was not yet so accentuated, the nutrient increase in water resulted as the principal driver in determining the long-term development of plankton communities and pelagic food web structure. Moreover, catchment-related processes may decisively affect both species composition and density of the lake planktonic communities due to the decrease of lake water transparency induced by input of inorganic material from the catchment area to the lake. The paleolimnological investigation, through the combined study of biotic and abiotic factor, allowed clarifying the synergic effects of the most important drivers of change in lake ecosystems, suggesting that climatic factors should be considered with nutrient availability as determinant element in controlling the temporal development of plankton communities and pelagic food web structure.

     

An Ecosystem Model for Exploring Lake Restoration Effects on Fish Communities and Fisheries in Florida

Developing quantitative ecosystem–scale expectations of habitat restoration projects and examining trade‐offs associated with alternative approaches has been a challenge for restoration ecology. Many of the largest freshwater lake restoration projects have occurred in Florida to remediate degradation to vegetated littoral habitats resulting from stabilized water levels, but effects across lake food‐webs have not been assessed. We developed an ecosystem model using Ecopath with Ecosim and Ecospace for a generalized large, eutrophic Florida Lake to explore how simulated restoration activities could influence fish communities with emphasis on sport fish abundance. We modeled three habitat restoration scenarios: (1) “no control,” (2) a “10‐year control” that restored littoral habitat every 10 years, and (3) a “combined control” scenario that restored littoral habitat every 10 years with maintenance controls between 10‐year periods. Our “combined control” scenario provided the largest long‐term habitat restoration benefits for sport fish abundance and the fisheries they support. In Ecospace, we simulated a littoral habitat restoration project that reduced lake‐wide tussock coverage from 30 to 15%. Ecospace predicted positive benefits to sport fish and fisheries following the restoration simulation and highlighted the importance of habitat edge effects, spatial design of habitat restoration projects, and sampling designs for evaluating restoration projects.     

湖泊水位变动对水生植被的影响机理及其调控方法

水位的高低及其变动范围、频率、发生的时间、持续的时长和规律性等是影响湖泊水生植被的核心因子.水位变动有短期、年内季节性和年际变动3种,对湖泊水生植被有不同的影响机理.水位短期变动通过对水体中的悬浮物、透明度、光衰减系数等的影响而对水生植被产生作用;周期性的年内季节性和年际水位变动可对水生植被的生态适宜性产生影响,并进而改变其时空分布;长期的高水位和低水位以及非周期性的水位季节变动会破坏水生植被长期以来对水位周期性变化所产生的适应性,从而影响了植被的正常生长、繁衍和演替.植被的极端深度和物种多样性是水位调控的核心表征指标,可通过经验数据分析法、生态模型法和参照法等方法来确定湖泊的适宜水位变动范围和时间.研究对象选择、研究方法、管理中的应用以及重要环境变化所产生的影响等是今后相关研究的核心问题.     

河湖生态系统生态用水优化研究——以滇池流域为例

生态需水是河流与湖泊生态系统健康的重要基础。湖泊流域的河流与湖泊生态系统之间存在密切的水量联系,目前对流域内生态用水的研究多为单一生态系统生态需水简单相加,忽略了河流和湖泊之间复杂的水量联系。基于河湖复合生态系统之间的水量联系构建了河湖生态系统生态用水优化模型,并以滇池为例分析了河湖生态系统生态用水规律。结果表明：湖泊流域中单一河流或湖泊生态需水计算结果不能满足复合生态系统的生态用水要求,需要综合考虑河流和湖泊之间的水量联系;在当前水质状况下,牛栏江每年的调水量不能满足滇池流域的生态用水要求;滇池流域水体污染对流域内生态用水影响较大,随着水体污染程度的下降,流域生态用水量和调水量呈指数下降,河流生态用水呈线性下降。     

Describing and understanding behavioral responses to multiple stressors and multiple stimuli

Understanding the effects of environmental change on natural ecosystems is a major challenge, particularly when multiple stressors interact to produce unexpected “ecological surprises” in the form of complex, nonadditive effects that can amplify or reduce their individual effects. Animals often respond behaviorally to environmental change, and multiple stressors can have both population‐level and community‐level effects. However, the individual, not combined, effects of stressors on animal behavior are commonly studied. There is a need to understand how animals respond to the more complex combinations of stressors that occur in nature, which requires a systematic and rigorous approach to quantify the various potential behavioral responses to the independent and interactive effects of stressors. We illustrate a robust, systematic approach for understanding behavioral responses to multiple stressors based on integrating schemes used to quantitatively classify interactions in multiple‐stressor research and to qualitatively view interactions between multiple stimuli in behavioral experiments. We introduce and unify the two frameworks, highlighting their conceptual and methodological similarities, and use four case studies to demonstrate how this unification could improve our interpretation of interactions in behavioral experiments and guide efforts to manage the effects of multiple stressors. Our unified approach: (1) provides behavioral ecologists with a more rigorous and systematic way to quantify how animals respond to interactions between multiple stimuli, an important theoretical advance, (2) helps us better understand how animals behave when they encounter multiple, potentially interacting stressors, and (3) contributes more generally to the understanding of “ecological surprises” in multiple stressors research.     

Anomalous rise in algal production linked to lakewater calcium decline through food web interactions

Increased algal blooms are a threat to aquatic ecosystems worldwide, although the combined effects of multiple stressors make it difficult to determine the underlying causes. We explore whether changes in trophic interactions in response to declining calcium (Ca) concentrations, a water quality issue only recently recognized in Europe and North America, can be linked with unexplained bloom production. Using a palaeolimnological approach analysing the remains of Cladocera (herbivorous grazers) and visual reflectance spectroscopically inferred chlorophyll a from the sediments of a Nova Scotia (Canada) lake, we show that a keystone grazer, Daphnia, declined in the early 1990s and was replaced by a less effective grazer, Bosmina, while inferred chlorophyll a levels tripled at constant total phosphorus (TP) concentrations. The decline in Daphnia cannot be attributed to changes in pH, thermal stratification or predation, but instead is linked to declining lakewater [Ca]. The consistency in the timing of changes in Daphnia and inferred chlorophyll a suggests top-down control on algal production, providing, to our knowledge, the first evidence of a link between lakewater [Ca] decline and elevated algal production mediated through the effects of [Ca] decline on Daphnia. [Ca] decline has severe implications for whole-lake food webs, and presents yet another mechanism for potential increases in algal blooms.     

Integrating vertical and horizontal approaches for management of shallow lakes and wetlands

Most lake restoration/rehabilitation schemes are biased toward vertical lake management practices generally applicable to deep lakes. Unfortunately, most schemes fail to or inadequately consider their actions within the context of horizontal lake management, an especially critical component when considering shallow lakes. Two Greek lakes, phytoplankton-dominated Koronia and macrophyte-dominated Chimaditida, are used to illustrate the importance of integrating vertical and horizontal considerations in the management of shallow lakes experiencing pronounced water level reduction. Attempting to manage the structure and function of fringing wetlands via vertical manipulations of the water column are doomed to failure without consideration of changes in physical and chemical aspects of the “memory” (sediments, soils). Fringing wetlands must not be considered as monotypic habitats interacting with lakes in direct proportion to their aerial extent. A predominately vertical lake management approach is probably valid for systems such as Lake Koronia without a history of significant submersed or emergent macrophytes. For those lakes embedded within significant wetlands like Lake Chimaditida, however, failure to consider horizontal lake management as a significant component of the overall system rehabilitation will likely diminish its successful outcome. Finally, definitions of wetlands currently used by Ramsar and aquatic scientists based primarily on structural aspects of ecosystems need to be modified to recognize the overriding importance of aerially differentiated functional aspects within vegetated communities as well as fundamental differences between vegetated and open-water habitats.     

长江中下游四大淡水湖生态系统完整性评价

长江中下游地区是我国淡水湖泊集中分布区域,研究该区域湖泊生态系统完整性对于湖泊生态系统保护和恢复具有重要意义。物理、化学和生物完整性指标已经广泛应用于河湖生态系统健康评价,但是缺少物理、化学和生物完整性的综合评价方法。以历史调查状况为主要参照系统,构建了基于物理、化学和生物完整性的多参数湖泊完整性综合评价指标体系,结合近年来长江中下游四大淡水湖(洞庭湖、鄱阳湖、巢湖、太湖)生态系统调查数据,对四大淡水湖生态系统完整性进行了评价。结果表明,洞庭湖、鄱阳湖、巢湖和太湖的综合得分分别为66、71、57和57。根据评价等级划分标准,洞庭湖和鄱阳湖生态系统完整性状况都达到"好"的等级,而巢湖和太湖则处于"一般"等级;结果显示,该指标能够表征人类活动对于湖泊生态系统完整性不同方面的干扰,且能够反映四大淡水湖生态系统完整性历史变化状况。因此,该方法可以作为长江中下游淡水湖泊生态系统完整性综合评价的工具并能够为湖泊生态系统的保护和恢复提供科学支撑。     

Porewater acid/base chemistry in near-shore regions of an acidic lake

Sediment porewaters in the near-shore region (within 1 m of the shoreline) of an acidic lake (Dart's Lake) were monitored during the summer of 1983 to investigate whether spatial variations in porewater acid/base chemistry were significant in this region of the lake. Previous investigations of Dart's Lake porewaters have indicated that within deeper waters (>2m depth), sediment porewaters are elevated in alkalinity relative to overlying lake water. Within the near-shore region, porewaters both considerably more and less acidic than the lake water were observed. Both reduction of strong acid anions (SO₄ ^2–, NO₃ ^–) and the mobilization of base cations were significant mechanisms of alkalinity production in porewaters exhibiting reducing conditions. In sediments reflecting oxic conditions, porewaters were generally more acidic than the lakewater. Measurement of groundwater seepage into the lake at the near-shore sites indicated that oxic sites exhibited elevated inputs of groundwater when compared to sites where reducing conditions existed. The acidic porewaters associated with high groundwater flows suggests that groundwater inputs to the lake may be a source of acidity (not alkalinity) on a whole-lake basis.     

A Model to Predict Lake Water Colour

This study is an attempt to quantitatively determine variables of significance for predicting colour in small glacial lakes. Lake colour is an important variable in many lake ecological contexts. The data emanate from two extensive data-sets from Sweden, one of which concerns 1456 lakes, and the other 91 more well-suited lakes. Four year average values of lake colour were compared to catchment and morphometric parameters to help identify the processes which influence variability in colour between lakes. Various hypotheses concerning the factors regulating colour in lakes were formulated and tested. Various statistical tests were used to separate random influences from causal influences. Those “map parameters” with the most significant influence on colour were the theoretical lake water retention time, the percent of rocks, lakes and mires of the drainage area, the ratio between lake area and drainage area and the lake mean depth. Each model parameter provides only a limited explanation (statistical) of the variability in colour between lakes. The predictability of colour by these models can not be markedly improved by accounting for the distribution of the characteristics in the drainage area. The stability of the final model, which gives an r²-value of 0.74, has been tested with positive results. The model allows lake colour to be estimated from knowledge of “geological” characteristics of the lake and its drainage area. The variability between lakes from other factors, such as temperature, precipitation and/or contamination of acidifying substances and nutrients, may then be quantitatively differentiated from the impact of these “geological” factors.     

洱海蓝藻爆发的时空特征及影响因子

湖泊生态环境对区域气候变化和流域人类活动十分敏感。随着流域的持续开发,洱海作为云贵高原第二大淡水湖泊面临着严重的生态与环境问题,主要包括水质恶化和生态功能的衰退,其中蓝藻水华问题尤为突出。针对湖泊现代监测数据存在时间序列较短、连续监测记录缺乏、监测位点不完全一致等问题,应用沉积物记录开展色素等多指标分析和环境变化重建研究,并对洱海湖区南、中、北3个湖盆的沉积物记录进行对比分析,从而探讨洱海富营养化与蓝藻爆发的历史与变化特征,并识别藻类响应模式的空间异同。沉积物色素记录结果表明,洱海蓝藻生物量变化具有明显的时空差异性,呈现由南至北,先后增加、最后呈现蓝藻水华全湖性持续爆发的模式。进一步的简约模型方差分解结果表明气候变暖和营养盐富集是洱海蓝藻生物量变化的主要驱动因子,此外相对较浅的南部湖盆还受到水位波动、水动力减弱、水生植物演化的综合影响。因此,在气候变暖的背景下,控制水体营养盐输入、合理调控湖泊水位、提高水体透明度并恢复水生植物是控制洱海蓝藻水华爆发和进行生态恢复的重要措施。     

First attempt to apply whole-lake food-web manipulation on a large scale in The Netherlands

Lake Breukeleveen (180 ha, mean depth 1.45 m), a compartment of the eutrophic Loosdrecht lakes system, was selected to study the effects of whole-lake foodweb manipulation on a large scale. In Lake Loosdrecht (dominated by filamentous cyanobacteria), due to water management measures taken from 1970–1984 (sewerage systems, dephosphorization) the external P load has been reduced from 1.2 g m⁻² y⁻¹ to 0.35 g m⁻² y⁻¹. The water transparency (Secchi-depthca. 30 cm), however, has not improved. The aim of the food-web manipulation in Lake Breukeleveen was not only to improve the light climate of the lake, but also to study if the successfull effects observed in small lakes (a few ha) can be upscaled. In March 1989 the standing crop of planktivorous and bentivorous fish populations was reduced by intensive fishery, fromca. 150 kg ha⁻¹ toca. 57 kg ha⁻¹. The lake was made unaccessible to fish migrating from the other lakes and it was stocked with large-sized daphnids and 0⁺ pike. However, water transparency did not increase in the following summer and autumn 1989, which is in contrast with great improvement in the light conditions previously observed in smaller lakes. The main explanations for the negative outcome in Lake Breukeleveen are: 1) the rapid increase of the planktivorous fish biomass and carnivorous cladocerans, predating on the zooplankton community; 2) suppression of the large daphnids by the high concentrations of filamentous cyanobacteria; 3) high turbidity of the lake due to resuspension of bottom material induced by wind, unlike in smaller lakes, and thus inability of submerged macrophytes to develop and to stabilize the ecosystem.     

Comparison of Cladocera-based water-depth reconstruction against other types of proxy data in Finnish Lapland

Lake depth is an important limnological attribute defining the structure and function of freshwater aquatic ecosystems. Lake levels have fluctuated and lake depths changed through the Holocene reflecting regional climate variations and sediment accumulation. Cladoceran remains preserved in sediments have been widely used for qualitative (P/L ratio) and quantitative (inference models) lake-depth reconstructions. In addition to estimations of prediction errors for performance power of modern data sets, it is important also to evaluate the reliability of reconstructed environmental values and to ensure that they are ecologically and paleoclimatically meaningful. In this study, we reconstructed the Holocene lake-depth history of a northern boreal lake using the Cladocera P/L ratio and a Cladocera—lake-depth inference model. These results were evaluated by comparison with reconstructions based on other proxies (aquatic macrofossils, sediment composition and sedimentation pattern) derived from three radiocarbon-dated sediment cores from the same lake. Whilst the reconstructions based on Cladocera and on the combination of other proxies yielded similar long-term trends, the absolute water depth values derived from the quantitative cladoceran model deviated from what was indicated by the other proxies. Therefore, we strongly recommend that also other, independent methods should be used simultaneously when reconstructing past water depths using Cladocera remains.     

Altered trophic pathway and parasitism in a native predator (Lepomis gibbosus) feeding on introduced prey (Dreissena polymorpha)

Populations of invasive species tend to have fewer parasites in their introduced ranges than in their native ranges and are also thought to have fewer parasites than native prey. This ‘release’ from parasites has unstudied implications for native predators feeding on exotic prey. In particular, shifts from native to exotic prey should reduce levels of trophically transmitted parasites. We tested this hypothesis in native populations of pumpkinseed sunfish (Lepomis gibbosus) in Lake Opinicon, where fish stomach contents were studied intensively in the 1970s, prior to the appearance of exotic zebra mussels (Dreissena polymorpha) in the mid-1990s. Zebra mussels were common in stomachs of present-day pumpkinseeds, and stable isotopes of carbon and nitrogen confirmed their importance in long-term diets. Because historical parasite data were not available in Lake Opinicon, we also surveyed stomach contents and parasites in pumpkinseed in both Lake Opinicon and an ecologically similar, neighboring lake where zebra mussels were absent. Stomach contents of pumpkinseed in the companion lake did not differ from those of pre-invasion fish from Lake Opinicon. The companion lake, therefore, served as a surrogate “pre-invasion” reference to assess effects of zebra mussel consumption on parasites in pumpkinseed. Trophically transmitted parasites were less species-rich and abundant in Lake Opinicon, where fish fed on zebra mussels, although factors other than zebra mussel consumption may contribute to these differences. Predation on zebra mussels has clearly contributed to a novel trophic coupling between littoral and pelagic food webs in Lake Opinicon.     

The photosynthetic and growth rate responses of two freshwater angiosperms in lakes of different trophic status: responses to light and dissolved inorganic carbon

SUMMARY 1. Two rooted freshwater macrophytes ( Lagarosiphon major (Ridley) Moss; Myriophyllum triphyllum Orchard) were grown in two lakes of differing trophic status on sediments collected from an eutrophic lake. In the two experiments (summer and winter) in the oligotrophic lake (Lake Taupo, New Zealand), the relative growth rates of both species were approximately double those recorded in the experiments in the eutrophic take (Lake Rotorua, New Zealand). These growth responses occurred even though the light and temperature regimes were similar at the experimental sites in both lakes and the concentrations of inorganic nutrients were higher in the eutrophic lake water.
2. Dissolved inorganic carbon (DIC) analysis of both lake waters indicated that the oligotrophic water had approximately 10-fold more DIC than the eutrophic water. This corresponded, at natural pH, to a 2-fold higher concentration of free-CO₂ in the oligotrophic lake water (49 μM) than in the eutrophic lake water (23 μM). Photosynthetic experiments demonstrated that the maximum rates of photosynthesis were 2.46 and 2.25-fold higher in L. major and M. triphyllum , respectively, when the shoots were incubated in Lake Taupo water at Lake Taupo levels of free-CO₂ relative to Lake Rotorua water at Lake Rotorua levels of free-CO₂.
3. It is concluded that these differences in photosynthetic rates would lead approximately to a 2-fold difference in growth rate, thus explaining the growth responses observed in the field experiments.