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Depth gradients in lakes are often key drivers of population divergence and speciation in fishes. New Zealand has many deep lakes but no known profundal specialist fishes or cases of intralacustrine speciation. We sampled a native benthic fish, the common bully (Gobiomorphus cotidianus), from 5 to 90 m depth in four South Island lakes to test for morphological, ecological, or genetic differentiation associated with depth. Deeper fish consistently had narrower bodies, while other morphological traits showed variable relationships with depth. Carbon and nitrogen stable isotope values of fish increased with depth, largely tracking isotopic trends with depth of benthic invertebrate prey. Genotyping-by-sequencing showed some genome-wide differentiation between two of the lakes, but no evidence for within-lake genetic structuring along depth gradients. These results indicate that individual bullies associate with shallower or deeper habitats within their lifetimes, but we found no evidence of progress toward genetic divergence within lakes. The apparent lack of intralacustrine genetic divergence in New Zealand's fishes may be explained by a combination of environmental factors and constraints intrinsic to its marine-derived freshwater fish fauna. Les gradients de profondeur dans les lacs sont souvent des facteurs cles de divergence des populations et de speciation chez les poissons. La Nouvelle-Zelande compte de nombreux lacs profonds, mais aucun poisson specialiste des profondeurs, ni cas de speciation intralacustre. Nous avons echantillonne un poisson benthique indigene, Gobiomorphus cotidianus, a des profondeurs allant de 5 a 90 m dans quatre lacs de l'lle du Sud, afin de verifier s'il presentait une differenciation morphologique, ecologique ou genetique associee a la profondeur. Les poissons preleves a de plus grandes profondeurs etaient uniformement caracterises par des corps plus etroits, alors que d'autres caracteres morphologiques presentaient des relations variables avec la profondeur. Les valeurs d'isotopes stables de carbone et d'azote des poissons augmentaient avec la profondeur, suivant generalement les tendances isotopiques en fonction de la profondeur de proies invertebrees benthiques. Le genotypage par sequenqage a fait ressortir une certaine differenciation a l'echelle du genome entre deux des lacs, mais aucun indice de structuration genetique a l'interieur d'un meme lac le long de gradients de profondeur. Ces resultats indiquent que des G. cotidianus sont tour a tour associes a des habitats de faible profondeur et de plus grande profondeur au cours de leur vie, mais aucun indice d'une progression vers une divergence genetique a l'interieur d'un meme n'a ete releve. L'apparente absence de divergence genetique intralacustre chez des poissons neo-zelandais pourrait s'expliquer par une combinaison de facteurs environnementaux et de contraintes intrinseques a la faune de poissons d'eau douce derives de poissons marins de Nouvelle-Zelande. [Traduit par la Redaction], Introduction Environmental gradients are important regulators of biodiversity, contributing to community turnover, population structure, and long-term evolutionary diversification (Doebeli and Dieckmann 2003; Seehausen et al. 2008). Water depth is a [...]

The eutrophication of lake ecosystems due to anthropogenic activities has impacted lakes worldwide and shallow lakes are particularly vulnerable. The retention of phosphorus (P) in lake sediments and its subsequent release to the water column is often a key factor in degraded water quality, algal blooms and delayed responses to lake restoration efforts. Despite this link between sedimentary P and poor ecosystem outcomes, a limited amount of research has been conducted on P dynamics in New Zealand's lakes. In this study, sediments in six shallow lakes across the country were analysed for geochemistry and P fractionation. Total sedimentary P content varied widely and was related to inflow concentrations and hydrological retention times. The P binding capacity of the sediments was related to iron (Fe) and aluminium (Al) concentrations, while the Fe:P ratio was correlated to the release of P under anoxic conditions. P fractions and release rates were poorly correlated with traditional water quality parameters, however the mobile-PSL and reducible-P fractions were strongly correlated with the frequency of summertime algal blooms. This relationship may provide an indicator to assess harmful algal bloom frequency in unmonitored lakes, and a valuable tool to assess internal loading risk and inform lake restoration methods. [ABSTRACT FROM AUTHOR]

Mysidae are widely distributed in aquatic ecosystems and form a large proportion of the hyperbenthic biomass in estuaries. The seasonality, spatial distribution and reproductive biology of the two hyperbenthic mysid species, Tenagomysis chiltoni and Tenagomysis novaezealandiae, were studied in an open estuary. Sampling was conducted bi-monthly at night between February 2008 and April 2009. These two mysid species are spatially segregated along a salinity gradient, with T. chiltoni upstream and T. novaezealandiae downstream in the estuary. The life cycle and breeding dynamics of T. chiltoni (bivoltine life cycle) and T. novaezealandiae (multivoltine life cycle) were similar to those described in intermittently closed estuaries, but their breeding period (December to April) was relatively shorter and densities were lower. Life-stage specific segregation in summer in both mysid species likely provided favourable conditions for maximum growth rates, short maturation times, and sheltering, enhancing reproductive success of both mysid species. [ABSTRACT FROM AUTHOR]

Lake snow, caused by the freshwater centric diatom Lindavia intermedia, has become problematic in several large, oligotrophic New Zealand lakes over the past decade. Macroaggregates produced by L. intermedia foul fishing lines, intake screens, and water filters, and have a negative impact on recreational values. It was confirmed that the fibers constituting lake snow are composed of chitin, two chitin synthase genes (chs1 and 2) from L. intermedia were characterized, new qPCR‐based tools to quantify the abundance of the species and measure expression of chs2 relative to the reference gene act1 (the product of which has cytoskeletal functions) were developed. The strong heterogeneity and mucilaginous nature of lake snow samples create particular difficulties for calibrations of gene or transcript copy numbers with cell densities and obtaining high yields of mRNA. However, data collected from four lakes during November 2018 and February and May 2019 show that abundance of L. intermedia is always high when lake snow is also abundant, but that a full range of L. intermedia abundance can occur when lake snow is absent, suggesting that chitin production is not obligate in L. intermedia. This result is consistent with the available data for chs2 expression, which suggest higher transcription when lake snow is abundant. Lake snow production by L. intermedia therefore requires an as yet undetermined stimulus independent of cell abundance. [ABSTRACT FROM AUTHOR]

To meet the challenges of preventing and reversing adverse effects of land use on ecosystems, management actions need to be founded on strong evidence. We used the pressure-state-impact (PSI) framework to assess evidence of land-use effects on New Zealand freshwater ecosystems. The evidence consisted of published quantitative and categorical associations linking land-use pressures to state changes and ecological impacts in rivers, lakes and aquifers. There was substantial evidence of land-use effects, particularly where land use/land cover (LULC) classes were used as pressure variables. Proportions of catchment area in urban and pastoral LULC were consistently, positively correlated with contaminant levels in water bodies and negatively correlated with ecological-health indicators. Other consistent PSI associations included positive correlations between cattle stocking rates and river contaminant levels, increased fine sediment and decreased ecological-health scores in rivers following forest harvest, and increased river contaminant levels at sites with stock access. Despite these consistent associations, the evidence base has four general shortcomings that should be addressed: (1) inadequate integration of data and models that link land use and contaminant loss to state changes and impacts in freshwater ecosystems; (2) weak inferences based on LULC; (3) reliance on categorical PSI associations; (4) gaps in reported PSI associations. [ABSTRACT FROM AUTHOR]

Scientists and environmental managers use stressor-response (S-R) relationships to characterise and predict ecological and socioeconomic responses to anthropogenic stressors in lakes and other aquatic ecosystems. S-R relationships with thresholds (stressor levels beyond which small increases cause large responses) are reported with increasing frequency. Threshold responses pose risks of unanticipated degradation, but also provide a stimulus for shifting environmental management strategies from retrospective (focused on post-hoc responses to degradation) to prospective (focused on preventing degradation). We set out a framework for interpreting S-R relationships in terms of functional forms, trajectories, thresholds and slopes. These characteristics convey information about resistance to degradation and recovery, risks of threshold exceedance, and alternate stable states. We then set out steps for implementing threshold-based management strategies, which are based on forecasting S-R relationships and carrying out preventative actions within an adaptive framework. Prospective, threshold-based management is a challenging yet promising alternative to the prevailing retrospective strategies. [ABSTRACT FROM AUTHOR]

Freshwater ecosystems are essential to peoples' economic, cultural, and social well‐being, yet are still among the most threatened ecosystems on the planet. Consequently, a plethora of recent regulations and policies seek to halt the loss of restore or safeguard freshwaters, their biodiversity, and the ecosystem services they provide. Ecosystem‐based management (EBM), an approach that considers human society as an integral part of ecosystems, is increasingly being promoted to help meet this challenge. EBM involves an overarching regulatory framework and local solutions with trade‐offs and compromises—factors that not only make decision processes complex but also provide the means for combining top–down regulation with bottom‐up priorities into collaborative management strategies. Although stakeholder participation is encouraged in most modern freshwater management, community values are often largely neglected. Here, we introduce a well‐known participatory decision support framework based on multicriteria decision analysis (MCDA) to operationalize EBM and promote community‐inclusive decision making in freshwater management. We explain the different steps that this approach comprises, which lead to the prioritization of a management strategy in a collaborative way. We also show how cultural values that inherently embed strong links between the environment and people can be used together with typical ecological and socio‐economic values. We illustrate the MCDA‐based EBM‐approach for New Zealand, one of the few countries in which regional freshwater management is mandated to uphold environmental quality standards, while safeguarding local community values and ecosystem services. Finally, we discuss some of the challenges, which are increasingly emerging as a result of mandated community collaboration in environmental management. [ABSTRACT FROM AUTHOR]