Highlights  The General Lake Model (GLM) is stress tested against 32 globally distributed lakes.  There was low correlation between input data uncertainty and model performance.  Model performance related to lake-morphometry, light extinction and flow regime; deep, clear lakes with high residence times had the lowest model error.

Ecosystems can show sudden and persistent changes in state despite only incremental changes in drivers. Such critical transitions are difficult to predict, because the state of the system often shows little change before the transition. Early-warning indicators (EWIs) are hypothesized to signal the loss of system resilience and have been shown to precede critical transitions in theoretical models, paleo-climate time series, and in laboratory as well as whole lake experiments. The generalizability of EWIs for detecting critical transitions in empirical time series of natural aquatic ecosystems remains largely untested, however. Here we assessed four commonly used EWIs on long-term datasets of five freshwater ecosystems that have experienced sudden, persistent transitions and for which the relevant ecological mechanisms and drivers are well understood. These case studies were categorized by three mechanisms that can generate critical transitions between alternative states: competition, trophic cascade, and intraguild predation. Although EWIs could be detected in most of the case studies, agreement among the four indicators was low. In some cases, EWIs were detected considerably ahead of the transition. Nonetheless, our results show that at present, EWIs do not provide reliable and consistent signals of impending critical transitions despite using some of the best routinely monitored freshwater ecosystems. Our analysis strongly suggests that a priori knowledge of the underlying mechanisms driving ecosystem transitions is necessary to identify relevant state variables for successfully monitoring EWIs.

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We present analysis of variations in relationships between nitrogen (N), phosphorus (P) and chlorophylla (chl-a) in lakes along a gradient of latitude inclusive of tropical, temperate and polar regions. Total nitrogen (TN), total phosphorus (TP), chl-a, latitude and depth data were collated for 1316 lakes situated between 70 ° S and 83 ° N. Latitudinal variation was then analysed for three empirical measures of phytoplankton nutrient limitation and/ or nutrient assimilation. Lastly, chl-a near-maxima conditional on TN and TP abundance were empirically defined for this global dataset using quantile regression. Mean TN:TP increases with distance from the equator. This relationship is independent of variation in either lake depth or trophic state, reflecting latitudinal variation in nutrient cycling processes and/or nutrient sources. There is a negative linear relationship between latitude and chl-a:TN which similarly suggests that N is less abundant relative to phytoplankton growth requirements at lower latitudes. Relative to temperate lakes, the statistical capability of TN and TP to predict chl-a is poor for both tropical and polar lakes, reflecting latitudinal variation in lake ecosystem functioning and the subsequent potential unsuitability of applying relationships derived for temperate lakes elsewhere. Chl-a near-maxima correspond to chl-a:TN and chl-a:TP yields of 0.046:1 and 0.87:1 respectively, although some observations greatly exceed near-maxima, suggesting possible physiologically plastic phytoplankton responses in these exceptional cases. Deficiencies in understanding the mechanisms that drive variation in macro-nutrient stoichiometry and phytoplankton biomass-nutrient relationships across large spatial scales necessitates further landscape-scale research on this topic, particularly in the tropics.

How climate change will affect the community dynamics and functionality of lake ecosystems during winter is still little understood. This is also true for phytoplankton in seasonally ice-covered temperate lakes which are particularly vulnerable to the presence or absence of ice. We examined changes in pelagic phytoplankton winter community structure in a north temperate lake (Müggelsee, Germany), covering 18 winters between 1995 and 2013. We tested how phytoplankton taxa composition varied along a winter-severity gradient and to what extent winter severity shaped the functional trait composition of overwintering phytoplankton communities using multivariate statistical analyses and a functional trait-based approach. We hypothesized that overwintering phytoplankton communities are dominated by taxa with trait combinations corresponding to the prevailing winter water column conditions, using ice thickness measurements as a winter-severity indicator. Winter severity had little effect on univariate diversity indicators (taxon richness and evenness), but a strong relationship was found between the phytoplankton community structure and winter severity when taxon trait identity was taken into account. Species responses to winter severity were mediated by the key functional traits: motility, nutritional mode, and the ability to form resting stages. Accordingly, one or the other of two functional groups dominated the phytoplankton biomass during mild winters (i.e., thin or no ice cover; phototrophic taxa) or severe winters (i.e., thick ice cover; exclusively motile taxa). Based on predicted milder winters for temperate regions and a reduction in ice-cover durations, phytoplankton communities during winter can be expected to comprise taxa that have a relative advantage when the water column is well mixed (i.e., need not be motile) and light is less limiting (i.e., need not be mixotrophic). A potential implication of this result is that winter severity promotes different communities at the vernal equinox, which may have different nutritional quality for the next trophic level and ecosystem-scale effects.

Here, we present a community perspective on how to explore, exploit and evolve the diversity in aquatic ecosystem models. These models play an important role in understanding the functioning of aquatic ecosystems, filling in observation gaps and developing effective strategies for water quality management. In this spirit, numerous models have been developed since the 1970s. We set off to explore model diversity by making an inventory among 42 aquatic ecosystem modellers, by categorizing the resulting set of models and by analysing them for diversity. We then focus on how to exploit model diversity by comparing and combining different aspects of existing models. Finally, we discuss how model diversity came about in the past and could evolve in the future. Throughout our study, we use Handling Editor: Piet Spaak.Electronic supplementary material The online version of this article (doi:10.1007/s10452-015-9544-1) contains supplementary material, which is available to authorized users. 123Aquat ) 49:513-548 DOI 10.1007 analogies from biodiversity research to analyse and interpret model diversity. We recommend to make models publicly available through open-source policies, to standardize documentation and technical implementation of models, and to compare models through ensemble modelling and interdisciplinary approaches. We end with our perspective on how the field of aquatic ecosystem modelling might develop in the next 5-10 years. To strive for clarity and to improve readability for non-modellers, we include a glossary.

A novel application of a continuous flow incubation system (CFIS) was used to assess four phosphorus (P) inactivation agents-alum, Phoslock TM , a new modified zeolite (Z2G1 or Aqual-P TM ), and allophone-when used as sediment capping agents to manage internal P loads in lakes. The CFIS technique allowed combined efficacy and sustainability assessment, including: (1) flux measurements during simulation of stratified (anoxic) and mixed (aerobic) conditions on the same sediment through multiple cycles to assess the longevity of a range of product doses; (2) simulation of a summer algal bloom collapse and subsequent burial of the products; and (3) investigation of non-target effects on nitrification and denitrification processes at the sediment-water interface. Minimum P-removal dose rates were found to differ substantially at 80 g m -2 for alum, 190 g m -2 for Z2G1, 220 g m -2 for allophane and 280 g m -2 for Phoslock TM , for similar capping layer thickness of about 2 mm, and would be effective for at least 4 years. All products temporarily suppressed nitrification and denitrification under aerobic conditions, and it may be important to minimise product application to any permanently aerobic zones, such as the littoral areas of a lake. While the aluminium (Al)-based products did not enhance Al fluxes in the CFIS, lanthanum (La) was released at a near constant rate of around 2 mg La m -2 day -1 from the Phoslock TM treatments over a period of at least 14 days. Spatial variability of sediment P, bioturbation, and burial are factors that will affect upscaling these results to a whole lake.

18Developing policies to address lake eutrophication requires an understanding of the 19 relative contribution of different nutrient sources and of how lake and catchment 20 characteristics interact to mediate the source-receptor pathway. We analysed total nitrogen 21 (TN) and total phosphorus (TP) data for 101 New Zealand lakes and related these to land 22 use and edaphic sources of P. We then analysed a sub-sample of lakes in agricultural 23 catchments to investigate how lake and catchment variables influence the relationship 24 between land use and in-lake nutrients. Following correction for the effect of covariation 25 amongst predictor variables, high producing grassland (intensive pasture) was the best 26 predictor of TN and TP, accounting for 38.6% and 41.0% of variation respectively. Exotic 27 forestry and urban area accounted for a further 18.8% and 3.6% of variation in TP and TN 28 respectively. Variation in mean catchment soil P could not account for variation in TP due 29 to the confounding effect of pastoral land use. Lake and catchment morphology (z max and 30 lake: catchment area) and catchment connectivity (lake order) mediated the relationship 31 between intensive pasture and in-lake nutrients. Mitigating eutrophication in New Zealand 32 lakes requires action to reduce nutrient export from intensive pasture and quantifying P 33 export from plantation forestry requires further consideration. 35Excess inputs of nitrogen (N) and phosphorus (P) to lakes can cause eutrophication and the 37 associated decline of water quality and ecological integrity (Vollenweider 1968; Smith 38 2003). Natural sources of these nutrients to freshwaters include: organic matter such as 39 plant residues which undergo mineralisation, atmospheric di-nitrogen fixed by 40 heterocystous phytoplankton species and P associated with apatite bearing minerals 41 (Newman 1995;Rabalais 2002). Inputs from anthropogenic sources are, however, 42 increasing in many parts of the world and loading associated with pollution now greatly 43 exceeds natural N and P loads to many lakes (Smith 2003). 44Lake managers require an understanding of nutrient sources and the processes that 45 drive lake productivity before developing plans to improve water quality in eutrophied 46 lakes (Moss 2007). Although limnologists have traditionally focussed on the study of in-47 lake processes (Johnes 1999), there is now widespread understanding that the successful 48 control of nutrient pollution and its associated problems is contingent on developing a 49 holistic and integrated understanding of lakes in the context of their wider catchments 50 (Ferrier and Jenkins 2010). While there is limited scope to adopt an experimental approach 51 to investigate how natural and anthropogenic factors influence nutrient loading to lakes, 52 the empirical analysis of relationships between lake and catchment variables across 53 different scales can advance mechanistic understanding. Geographical Information 54 Systems (GIS) can provide a platform for the collation an...