Jonathan Michael Abell

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.

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.

Reducing the external environmental costs of pastoral farming in New Zealand: experiences from the Te Arawa lakes, Rotorua

Decades of nutrient pollution have caused water quality to decline in the nationally iconic Te Arawa (Rotorua) lakes in New Zealand. Pastoral agriculture is a major nutrient source, and therefore this degradation represents an external environmental cost to intensive farming. This cost is borne by the wider community, and a major publically funded remediation programme is now under way. This article describes the range of actions being taken to reduce nutrient loads from internal (lake bed sediments) and external (primarily diffuse) sources in the lake catchments. The high economic cost and uncertain efficacy of engineering-based actions to reduce internal nutrient loads is highlighted. Major changes to land management practices to control diffuse nutrient pollution are required throughout New Zealand if the need for costly and lengthy remediation programmes elsewhere is to be avoided. More action to educate farmers and the public about eutrophication issues, development and enforcement of environmental standards, and further consideration of the use of market-based instruments are proposed as ways to correct the current market failure.

Latitudinal variation in nutrient stoichiometry and chlorophyll-nutrient relationships in lakes: A global study

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.

Quantifying temporal and spatial variations in sediment, nitrogen and phosphorus transport in stream inflows to a large eutrophic lake

High-frequency sampling of two major stream inflows to a large eutrophic lake (Lake Rotorua, New Zealand) was conducted to measure inputs of total suspended sediment (TSS), and fractions of nitrogen and phosphorus (P). A total of 17 rain events were sampled, including three during which both streams were simultaneously monitored to quantify how concentration-discharge (Q) relationships varied between catchments during similar hydrological conditions. Dissolved inorganic nitrogen (DIN) concentrations declined slightly during events, reflecting dilution of groundwater inputs by rainfall, whereas dissolved inorganic P (PO4-P) concentrations were variable and unrelated to Q, suggesting dynamic sorptive behaviour. Event loads of total nitrogen (TN) were predominantly DIN, which is available for immediate uptake by primary producers, whereas total phosphorus (TP) loads predominantly comprised particulate P (less labile). Positive correlations between Q and concentrations of TP (and to a lesser extent TN) reflected increased particulate nutrient concentrations at high flows. Consequently, load estimates based on hourly Q during storm events and concentrations of routine monthly samples (mostly base flow) under-estimated TN and TP loads by an average of 19% and 40% respectively. Hysteresis with Q was commonly observed and inclusion of hydrological variables that reflect Q history in regression models improved predictions of TN and TP concentrations. Lorenz curves describing the proportions of cumulative load versus cumulative time quantified temporal inequality in loading. In the two study streams, 50% of estimated two-year loads of TN, TP and TSS were transported in 202-207, 76-126 and 1-8 days respectively. This study quantifies how hydrological and landscape factors can interact to influence pollutant flux at the catchment scale and highlights the importance of including storm transfers in lake loading estimates.

Phosphorus and nitrogen loading restraints are essential for successful eutrophication control of Lake Rotorua, New Zealand

Abstract Anthropogenic activity has greatly enhanced the inputs of nitrogen (N) and phosphorus (P) to lakes, causing widespread eutrophication. Algal or cyanobacterial blooms are among the most severe consequences of eutrophication, impacting aquatic food webs and humans that rely on lakes for ecosystem services. In New Zealand, recent debate on the relative importance of N versus P control for limiting occurrences of algal blooms has centered on the iconic Lake Rotorua (North Island). Water quality in Lake Rotorua has declined since the late 1800s following catchment vegetation clearing and subsequent land-use intensification, as well as from sewage inputs. A multimillion dollar restoration programme began in the early 2000s, with key mitigation actions including nutrient load targets for the entire catchment and alum dosing in 2 tributaries. In this manuscript we analyse 2 water quality datasets (>10 yr) from Lake Rotorua and compare these with a global lake dataset. Generalised additive models predicted highly significant (p < 0.001) declines in total phosphorus (TP), total nitrogen (TN) and chlorophyll a (Chl- a) in surface waters between 2001 and 2015. Alum dosing had a negative (i.e., reducing) and highly significant effect on TP and Chl- a (p < 0.001). Correlations of Chl- a on TP and TN were highly significant, but the difference between the 2 correlation coefficients was not, indicating a need to control both nutrients to reduce algal productivity. This conclusion is reinforced by recent bioassay studies which show co-limitation by N and P. Collectively, our data and previous studies provide strong support for the current strategy of limiting both N and P loads to Lake Rotorua for effective eutrophication control.

Bioavailability of phosphorus transported during storm flow to a eutrophic, polymictic lake

The bioavailability of catchment-derived particulate phosphorus (PP) to lentic phytoplankton was investigated. Two stream inflows to Lake Rotorua (Bay of Plenty, New Zealand) were sampled during moderate (March) and large (August) discharge peaks in 2012. Phosphorus (P) fractions in samples were quantified and a bioassay was undertaken using samples from the March event to determine whether P-limited phytoplankton could utilise PP. Phosphorus composition differed markedly between events but was comparable between streams. For the March event, all PP (persulphate extractable) was bioavailable if exposed to anoxia, whereas c. 25% was potentially bioavailable for the August event. Despite this, the laboratory bioassay indicated that, under oxic conditions, suspended sediments acted as a sink rather than a source of bioavailable P to P-limited phytoplankton. This study highlights the importance of considering P speciation and the nature of the receiving environment when assessing the bioavailability of P transported through hydrological landscapes.

Modelling hydrology and water quality in a mixed land use catchment and eutrophic lake: Effects of nutrient load reductions and climate change

Abstract The objective of this study was to combine a catchment model with a one–dimensional lake water quality model to simulate the trophic state of a eutrophic shallow lake in response to nutrient load reductions and climate change. The catchment and lake models gave satisfactory performance in simulating observed data, indicating that the key processes that affect nutrient loads and lake trophic status were adequately represented. Simulating removal of nutrients by reducing fertiliser applied to farmland or irrigated wastewater had minor effects on nutrient concentrations in the lake, but simulations using a projected climate for 2090 showed a major impact on nutrients and water quality. This overarching effect indicated that polymictic lakes may be particularly vulnerable to eutrophication associated with climate change due to increased internal nutrient loading, which will lead to a biological response of increased algal biomass, while changes in external loads will have lesser relative impact.

Wind forced circulation and sediment disturbance in a temperate lake

ABSTRACT Meteorological forcing on the surface of a lake can generate mixing and transport processes that alter water quality. This study examined how meteorological forcing affected water currents in Lake Rotorua—a large, shallow, temperate, polymictic lake—during a period of sustained vertical mixing and strong winds. Acoustic Doppler current profiler (ADCP) measurements were recorded at two sites and a hydrodynamic model (ELCOM) was used to simulate water movement over the period of ADCP measurements. Model simulations show circulation patterns in Lake Rotorua are strongly influenced by the topography of the lake and, to a lesser extent, the catchment, with little variation in currents with depth during winter mixing. Northeasterly winds produced a clockwise circulation around the mid-lake island with a second anticlockwise gyre between the island and the western shore. This circulation pattern is consistent with topographic gyres found in other large lakes. A shift to a weaker southwesterly wind caused these circulation patterns to reverse direction over a period of c. 6 h, with a weak secondary gyre also occurring within the main anticlockwise circulation. The effect of this circulation pattern on the resuspension of sediment and thus potential in-lake phosphorus control strategies is considered.