Speciation and Dynamics of Phosphorus in Relation to Lake Restoration Methods

Lake Okaro is a small, warm monomictic lake in the central North Island of New Zealand. It has remained highly eutrophic despite an intensive catchment and in-lake restoration program which commenced in 2003. The program has included the implementation of a constructed wetland, riparian protection, an alum application and application of a modified zeolite mineral (Z2G1) to reduce internal nutrient loading. This study examines water column and sediment nutrient dynamics; focusing on phosphorus (P) and the ecosystem response to lake restoration designed to reduce levels of P. Trends in P concentrations in Lake Okaro were linked to the restoration efforts over a six-year period (2002-08) including the period shortly before the restoration program. Over the entire study period, the annual average total phosphorus (TP) concentration in the lake decreased by 56 %. Two predictive models, which derive the annual average P concentration in the water column based on external P loading, generally underestimated the measured TP concentrations in the water column due to internal P loading. Of all restoration methods, the application of Z2G1 produced the most effective reduction in water column TP concentrations. However the lake trophic state showed high resilience to reduced internal P loading even though the combined effect of all restoration procedures resulted in significantly decreased TP concentrations in the lake. The sources and sinks of nutrients in the hypolimnion of Lake Okaro were investigated using field measurements in a comprehensive nutrient budget model in order to determine changes in sediment nutrient fluxes resulting from a whole lake sediment capping trial using Z2G1. Sediment nutrient fluxes in the hypolimnion were estimated as the residual term in the nutrient budget model that accounted for mineralisation of organic nutrients, nutrient uptake by phytoplankton, nitrification, adsorption or desorption of P from inorganic particulate material in the water column, and diffusion of dissolved nutrients at the thermocline. The model indicated that during a period of seasonal stratification in 2007-08 up to 60% of hypolimnetic phosphate fluxes and 50% of ammonium fluxes were derived from bottom sediments. Diffusion across the thermocline, adsorption/desorption of phosphate to suspended solids, and nitrification were of relatively minor importance (less than or equal to 9%) to the total fluxes. Any reduction in sediment nutrient release by Z2G1 was small compared with both the total sediment nutrient flux and the sum of other hypolimnetic fluxes. Sediment and settling seston organic P composition was determined using 31P nuclear magnetic resonance (NMR). Settling seston and sediment samples were analysed during winter and summer, representing, respectively, a mixing period when the water column was well oxygenated and a stratified period when the hypolimnion was anoxic. The bottom sediments and settling seston contained orthophosphate, orthophosphate mono- and diesters, pyrophosphates, polyphosphates, and phosphonates with organic P content exceeding 60% of the total extracted P occasionally. Phosphorus content in settling seston increased 2.5-fold in winter, with a marked increase in orthophosphate content. The 31P NMR analyses revealed the presence of several potentially bioavailable P compounds, which may be recycled from the sediment to the water column. An apparent half-life‟ value was used to quantify the time scales on which these compounds are degraded within the sediment and likely being recycled to the overlying water column. Relatively long half-life values, ranging from 8 to 23 years, indicate that this recycling could potentially reduce the efficacy and longevity of in-lake restoration procedures that have been applied to Lake Okaro. A one-dimensional process based ecosystem model (DYRESM-CAEDYM) was used to simulate the potential effect on water quality of Lake Okaro of separate and combined reductions in external and internal loads of nitrogen (N) and P. The model was calibrated against field data for a two-year period and validated over two separate one-year periods including a year immediately following a Z2G1 application and a year when there was an extraordinary algal bloom from an invasive, highly buoyant, N-fixing cyanobacterium, Anabaena planktonica. The model simulations reproduced the scale of phosphate and ammonium concentrations at 14 m depth, corresponding to the deeper region of the hypolimnion, both before and after the application of Z2G1, with no adjustment of parameters, suggesting that there was little effect of the Z2G1, at least within the uncertainties of the model runs. The model simulations were less successful in reproducing the Anabaena planktonica bloom. This was attributed to a lake of flexibility in the conceptualisation and calibration of the model, which meant that it could not encompass this invasive species. In the model scenarios with reduced nutrient loading, the trophic status of Lake Okaro, given quantitatively by the Trophic Level Index (TLI), decreased to a greater extent with a given fractional reduction of the internal load than a reduction of the external load. The control of both N and P was shown in simulations to be more effective in reducing phytoplankton biomass than for N or P alone, tending to affirm an N+P control paradigm. Undesirable shifts in zooplankton and phytoplankton species composition due to the application of Z2G1 were investigated by comparing the plankton community structure before and after the Z2G1 application. No significant differences in species composition were found at the depths investigated (surface and 9 m). However, further analyses showed statistically significant differences between seasons, indicating that seasonal variations in plankton composition far outweighed changes that occurred as a result of the Z2G1 application. In this study, field measurements and numerical modelling provided a comprehensive assessment methodology of testing the response of Lake Okaro to reduced nutrient loading, with a focus on P dynamics. Although nutrient loads to Lake Okaro were reduced using catchment and in-lake restoration methods, further substantial and prolonged reduction in both N and P loading appear to be required to decrease phytoplankton biomass and the trophic state of the lake. This study highlights the need for investigating water column P content and sediment P composition for evaluating the potential magnitude of internal loading, and emphasises the importance of using process based numerical modelling as a decision support tool for lake management

Modelling the response of a highly eutrophic lake to reductions in external and internal nutrient loading

The reduction of macronutrients to levels that limit primary production is often a critical element of mitigating eutrophication and reducing the potential for algal blooms. Lake Okaro has remained highly eutrophic despite an intensive catchment and in-lake restoration programme, including implementation of a constructed wetland, riparian protection, an alum application and application of a modified zeolite mineral (Z2G1) to reduce internal nutrient loading. A one-dimensional process-based ecosystem model (DYRESM-CAEDYM) was used in this study to investigate the need for further nutrient loading reductions of both nitrogen (N) and phosphorus (P). The model was calibrated against field data for a 2-year period and validated over two separate 1-year periods. Model simulations suggest that the trophic status of the lake, measured quantitatively with the Trophic Level Index (TLI), could shift from highly eutrophic to mesotrophic with external and internal loads of both N and P reduced by 75-90%. The magnitude of the nutrient load reductions is indicative of a major challenge in being able to effect transitions across trophic state categories for eutrophic lakes

Nitrogen and phosphorus limitation of phytoplankton growth in New Zealand lakes: Implications for eutrophication control

We examine macronutrient limitation in New Zealand (NZ) lakes where, contrary to the phosphorus (P) only control paradigm, nitrogen (N) control is widely adopted to alleviate eutrophication. A review of published results of nutrient enrichment experiments showed that N more frequently limited lake productivity than P; however, stoichiometric analysis of a sample of 121 NZ lakes indicates that the majority (52.9%) of lakes have a mean ratio of total nitrogen (TN) to total phosphorus (TP) (by mass) indicative of potential P-limitation (>15:1), whereas only 14.0% of lakes have mean TN:TP indicative of potential N-limitation (<7:1). Comparison of TN, TP, and chlorophyll a data between 121 NZ lakes and 689 lakes in 15 European Union (EU) countries suggests that at the national scale, N has a greater role in determining lake productivity in NZ than in the EU. TN:TP is significantly lower in NZ lakes across all trophic states, a difference that is driven primarily by significantly lower in-lake TN concentrations at low trophic states and significantly higher TP concentrations at higher trophic states. The form of the TN:TP relationship differs between NZ and the EU countries, suggesting that lake nutrient sources and/or loss mechanisms differ between the two regions. Dual control of N and P should be the status quo for lacustrine eutrophication control in New Zealand and more effort is needed to reduce P inputs

Re-using DSpace to build a repository for freshwater quality data

This presentation describes how we are adapting DSpace to build LERNZdb, a repository for storing and disseminating New Zealand freshwater quality data. While the original intention in this project was to build a database for individual measurements, the repository model turned out to be a very good starting point for meeting the requirements. This allowed us to re-use our DSpace expertise gained from working on institutional publications repositories. After a quick introduction to LERNZdb's aims, data and users, the main emphasis of this presentation is on our DSpace modifications

Relationships between land use and nitrogen and phosphorus in New Zealand lakes

Developing policies to address lake eutrophication requires an understanding of the relative contribution of different nutrient sources and of how lake and catchment characteristics interact to mediate the source–receptor pathway. We analysed total nitrogen (TN) and total phosphorus (TP) data for 101 New Zealand lakes and related these to land use and edaphic sources of phosphorus (P). We then analysed a sub-sample of lakes in agricultural catchments to investigate how lake and catchment variables influence the relationship between land use and in-lake nutrients. Following correction for the effect of co-variation amongst predictor variables, high producing grassland (intensive pasture) was the best predictor of TN and TP, accounting for 38.6% and 41.0% of variation, respectively. Exotic forestry and urban area accounted for a further 18.8% and 3.6% of variation in TP and TN, respectively. Soil P (representing naturally-occurring edaphic P) was negatively correlated with TP, owing to the confounding effect of pastoral land use. Lake and catchment morphology (zmax and lake : catchment area) and catchment connectivity (lake order) mediated the relationship between intensive pasture and in-lake nutrients. Mitigating eutrophication in New Zealand lakes requires action to reduce nutrient export from intensive pasture and quantifying P export from plantation forestry requires further consideration

Restoration of Lake Hakanoa: Results of model simulations

This report was requested by Waikato District Council. It covers the lake water quality of, and possible restoration scenarios for, Lake Hakanoa a riverine lake situated in Huntly. The lake is used as a recreational resource by the community. In the past it has been reported to have had very poor water quality and is known to be eutrophic. It is currently in an algal-dominated, devegetated state and has low water clarity. The shallowness of this lake makes it potentially susceptible to resuspension of sediments through wind action. A community group, Friends of Hakanoa, has been responsible for the formation of a path around the perimeter of the lake, retiring about 3.6% of the catchment from pastoral farming and creating a riparian margin. Results from more recent reports and this report indicate a trend of improving water quality which may be related to recent restoration actions such as re-establishment of a riparian margin

Parameterisation of sediment geochemistry for simulating water quality responses to long-term catchment and climate changes in polymictic, eutrophic Lake Rotorua, New Zealand

Numerical models of aquatic ecosystems that couple physics and biogeochemistry are valuable tools in aquatic ecosystem research. These models provide opportunities to test theories and to inform environmental management. In this study, we used the dynamic, process-based hydrodynamic-ecological model DYRESM-CAEDYM to simulate key ecosystem processes of Lake Rotorua, New Zealand, for six 8-year periods between 1920 and 2100 in order to evaluate the potential effects of future changes in land use and climate. Longterm variations in external boundary conditions (e.g. inflows) to the lake ecosystem are incorporated by varying the relevant input files in the DYRESMCAEDYM model. However, quantification of internal lake processes, specifically those at the sediment-water interface, presents a major challenge for long-term simulations. The sediment model within CAEDYM is ‘static’, with assumed constant sediment composition and a relatively simplistic process representation for nutrient and oxygen fluxes between sediment and water. Specifically, the model regulates sediment phosphate and ammonium release according to concentrations of oxidising species (i.e. oxygen and nitrate), and temperature in the overlying water layer. Sediment oxygen demand is controlled by dissolved oxygen concentrations and temperature in the water layer overlying the sediments. We used a ‘trial and error’ approach to estimate parameters for calibrating and validating the model, and regression modelling to infer the parameters beyond the calibration/validation simulation period (2001–2009). We observed a significant relationship in historic monitoring data between the external nitrogen load to the lake and its hypolimnetic oxygen demand as well as the bottom-sediment nitrogen concentrations. This relationship was used to hindcast and forecast model parameters for sediment nutrient release and oxygen demand in the six model simulation periods. The inclusion of a dynamic response of sediment nutrient release and oxygen demand parameters to changes in external nutrient loads enabled a more conceptually concise simulation of water quality for the simulations. This model is currently being used by regional environmental management authorities for developing an Action Plan for the restoration of Lake Rotorua

Effects of water temperature on summer periphyton biomass in shallow lakes: a pan-European mesocosm experiment

Periphyton communities play an important role in shallow lakes and are controlled by direct forces such as temperature, light, nutrients, and invertebrate grazing, but also indirectly by planktivorous fish predation. We performed a pan-European lake mesocosm experiment on periphyton colonization covering five countries along a north/south geographical/temperature gradient (Estonia, Germany, Czech Republic, Turkey, and Greece). Periphyton biomass on artificial polypropylene strips exposed at 50 cm water depth at low and high nutrient regimes (with mean total phosphorus concentration of 20 and 65 µg L−1, respectively) was compared during mid-summer. No significant effect of nutrient loading on periphyton biomass was observed as nutrient concentrations in the mesocosms were generally above limiting values. Water temperature significantly enhanced summer periphyton biomass development. Additionally, direct and indirect top-down control of snails and fish emerged as a significant factor in periphyton biomass control

Review of best management practices for aquatic vegetation control in stormwater ponds, wetlands, and lakes

Auckland Council (AC) is responsible for the development and operation of a stormwater network across the region to avert risks to citizens and the environment. Within this stormwater network, aquatic vegetation (including plants, unicellular and filamentous algae) can have both a positive and negative role in stormwater management and water quality treatment. The situations where management is needed to control aquatic vegetation are not always clear, and an inability to identify effective, feasible and economical control options may constrain management initiatives. AC (Infrastructure and Technical Services, Stormwater) commissioned this technical report to provide information for decision- making on aquatic vegetation management with in stormwater systems that are likely to experience vegetation-related issues. Information was collated from a comprehensive literature review, augmented by knowledge held by the authors. This review identified a wide range of management practices that could be potentially employed. It also demonstrated complexities and uncertainties relating to these options that makes the identification of a best management practice difficult. Hence, the focus of this report was to enable users to screen for potential options, and use reference material provided on each option to confirm the best practice to employ for each situation. The report identifies factors to define whether there is an aquatic vegetation problem (Section 3.0), and emphasises the need for agreed management goals for control (e.g. reduction, mitigation, containment, eradication). Resources to screen which management option(s) to employ are provided (Section 4.0), relating to the target aquatic vegetation, likely applicability of options to the system being managed, indicative cost, and ease of implementation. Initial screening allows users to shortlist potential control options for further reference (Section 5.0). Thirty-five control options are described (Section 5.0) in sufficient detail to consider applicability to individual sites and species. These options are grouped under categories of biological, chemical or physical control. Biological control options involve the use of organisms to predate, infect or control vegetation growth (e.g. classical biological control) or manipulate conditions to control algal growth (e.g. pest fish removal, microbial products). Chemical control options involve the use of pesticides and chemicals (e.g. glyphosate, diquat), or the use of flocculants and nutrient inactivation products that are used to reduce nutrient loading, thereby decreasing algal growth. Physical control options involve removing vegetation or algal biomass (e.g. mechanical or manual harvesting), or setting up barriers to their growth (e.g. shading, bottom lining, sediment capping). Preventative management options are usually the most cost effective, and these are also briefly described (Section 6.0). For example, the use of hygiene or quarantine protocols can reduce weed introductions or spread. Catchment- based practices to reduce sediment and nutrient sources to stormwater are likely to assist in the avoidance of algal and possibly aquatic plant problems. Nutrient removal may be a co-benefit where harvesting of submerged weed biomass is undertaken in stormwater systems. It should also be considered that removal of substantial amounts of submerged vegetation may result in a sudden and difficult-to-reverse s witch to a turbid, phytoplankton dominated state. Another possible solution is the conversion of systems that experience aquatic vegetation issues, to systems that are less likely to experience issues. The focus of this report is on systems that receive significant stormwater inputs, i.e. constructed bodies, including ponds, amenity lakes, wetlands, and highly-modified receiving bodies. However, some information will have application to other natural water bodies

Reference and current Trophic Level Index of New Zealand lakes: benchmarks to inform lake management and assessment

Knowledge of trophic status is fundamental to understanding the condition and function of lake ecosystems. We developed regression models to predict chlorophyll a concentrations (chl a) in New Zealand lakes for reference and current states, based on an existing dataset of total nitrogen (TN) and total phosphorus (TP) concentrations for 1031 lakes. Models were then developed to predict Secchi depth based on chl a and a sediment resuspension term applicable to shallow lakes. Estimates of all four Trophic Level Index (TLI) variables (chl a, TN, TP and Secchi depth) were analysed to estimate reference and current state TLI for the nationally representative sample of 1031 lakes. There was a trend of eutrophication between reference and current states, with systematic differences among lake geomorphic types. Mean chl a increased 3.5-fold (2.42 mg m­¯­­­³ vs. 8.32 mg m¯³) and mean Secchi depth decreased (indicating lower clarity) by approximately one-third (9.62 m vs. 6.48 m) between reference and current states. On average, TLI increased by 0.67, with the TLI increase >1 in approximately one-third (31%) of lakes. This study informs the status of lake ecosystems in NZ and provides benchmarks to guide management and assessment