Clive Howard-Williams

Estuaries are unique ecosystems where fresh and salt waters mix, connecting land to the ocean. They can take many forms (Hume et al. 2016) and are known by many names, including wahapū, harbours, l...

The ecological effects of wastewater treatment plant (WWTP) discharges into rivers typically include elevated periphyton biomass resulting from enrichment by dissolved reactive phosphorus (DRP), dissolved inorganic nitrogen (DIN), or both. WWTP discharges may transfer DIN to receiving water as ammonium-N (NH₄-N) and, following nitrification steps, nitrate-N (NO₃-N). The potential for NH₄-N to exacerbate nuisance periphyton biomass compared to NO₃-N is not well understood. Theoretically, NH₄-N is the more energy-efficient source of N for algae, but its effect on algal growth rates depends on concentration, interaction with NO₃-N, and environmental conditions, such as temperature. We investigated the relative importance of NH₄-N and NO₃-N in WWTP discharges by tracking periphyton biomass and algal assemblage composition in streamside channels for 27 d under 500 µg DIN/L comprising different proportions of NH₄-N: 0.5% (low-NH₄, ambient levels of NH₄-N, 2.2 µg/L), 30% (medium-NH₄, 160 µg/L), and 77% NH₄-N (high-NH₄, 389 µg/L). The DIN treatments had replete DRP (25 µg/L) and were compared to controls with no enrichment (control) and DRP enrichment only (P-control). Biomass was measured as chlorophyll a (Chl a) and ash-free dry mass (AFDM), which showed contrasting responses to different proportions of NH₄-N enrichment. Chl a was higher under high-NH₄ than under low-NH₄ (p < 0.01, 50% higher at maximum Chl a on d 20). Under medium-NH₄ and low-NH₄, Chl a was not different from the controls and P-controls (p > 0.15). AFDM did not differ among the 3 DIN treatments (p > 0.11) but was higher in all DIN treatments than in the controls (p < 0.05). Algal assemblage composition discriminated the controls from all other treatments and the low-NH₄ from high-NH₄ treatments. The assemblage analyses indicated that high-NH₄ favored enhanced cell division rates in small diatoms, especially Encyonema minutum. Higher Chl a/unit volume of small taxa than larger taxa likely explained elevated Chl a under high-NH₄. The results illustrated that, with replete DRP, elevating DIN through enrichment with NH₄-N may cause additional increases in periphyton biomass as Chl a compared to elevating DIN through NO₃-N enrichment. Consequently, enhanced management to specifically reduce NH₄-N outputs in WWTP discharges may assist in the control of nuisance periphyton downstream.

(2020). Preface: land-use impacts on aquatic ecosystems - tribute to John Quinn. New Zealand Journal of Marine and Freshwater Research: Vol. 54, Special issue: Land-use impacts on aquatic ecosystems: evidence and solutions —Tribute to John Quinn, pp. 305-307.

Consideration of the need and opportunities for restoration of freshwater lakes in New Zealand requires individualised approaches that account for lake type, invasive species and food web structure.

The coastal zone worldwide is not well served by management policies that fail to deal effectively with land-sourced contaminants from streams, rivers and urban runoff. We discuss this using examples from New Zealand, where there is a wide recognition of such problems but little effective policy that specifically accounts for the interconnectedness of the land-to-freshwater-to-sea domain. Increasing land-use intensification, mostly in arable pasture, has greatly increased the nutrient load to the coast and has contributed to the already high sediment loading of coastal waters. We argue that renewed effort is needed for a more holistic approach to management, encapsulated in the New Zealand Maori concept of ki uta ki tai – an appreciation of mountains-to-sea connections. Limit-setting, especially of nitrogen from terrestrial sources, seems to a main way forward for reducing loads and halting cumulative effects. However, this must account for current impacts and also the ‘load to come’ because of time lags associated with accumulated groundwater nutrients and cumulative impacts in receiving waters. Despite many approaches being tried, there are increasing impacts of catchment activities on coasts worldwide. The need is therefore great for new approaches, renewed effort, and for a very long-term perspective to ensure effective and enduring solutions.