Search

Continental shelf sediments are considered hotspots for nitrogen (N) removal. While most investigations have quantified denitrification in shelves receiving large amounts of anthropogenic nutrient supply, we lack insight into the key drivers of N removal on oligotrophic shelves. Here, we measured rates of N removal through denitrification and anammox by the revised‐isotope pairing technique (r‐IPT) along the Northeastern New Zealand shelf. Denitrification dominated total N2 production at depths between 30 and 128 m with average rates (± SE) ranging from 65 ± 28 to 284 ± 72 μmol N m−2 d−1. N2 production by anammox ranged from 3 ± 1 to 28 ± 11 μmol N m−2 d−1 and accounted for 2–19% of total N2 production. DNRA was negligible in these oligotrophic settings. Parallel microbial community analysis showed that both Proteobacteria and Planctomycetota were key taxa driving denitrification. Denitrification displayed a negative correlation with oxygen penetration depth, and a positive correlation with macrofauna abundance. Our denitrification rates were comparable to oligotrophic shelves from the Arctic, but were lower than those from nutrient‐rich Pacific and Atlantic shelves. Based on our results and existing IPT measurements, the global shelf denitrification rate was reassessed to be 53.5 ± 8.1 Tg N yr−1, equivalent to 20 ± 2% of marine N removal. We suggest that previous estimates of global shelf N loss might have been overestimated due to sampling bias toward areas with high N loads in the Northern Hemisphere. [ABSTRACT FROM AUTHOR]

Commercial harvesting often reduces densities of target species, potentially affecting transmission of parasites within the population. Rates of parasitic infection not only impact the hosts directly, but can result in wider ecosystem effects through influences on host behaviour. Commercial harvesting may therefore have important ecological consequences beyond the direct effects of removal of biomass. Clams (Austrovenus stutchburyi) are commonly infected by trematode parasites (Acanthoparyphium spp. and Curtuteria australis) and have been commercially harvested in Otago, New Zealand, since 1983. To assess the effects of harvesting on the abundance of trematodes in A. stutchburyi, the number of parasites per clam was compared in harvested and unharvested areas. The mean number of parasites per clam was 36% higher at commercially harvested sites, suggesting that harvesting enhances the local transmission of parasites. Harvesters may be able to reduce their influence on parasite infection levels in clam populations by harvesting less intensively and allowing more time between harvests for clam biomass to regenerate. L'exploitation commerciale reduit souvent la densite des especes visees, ce qui peut avoir une incidence sur la transmission de parasites au sein des populations. Les taux d'infection parasitaire n'ont pas qu'un impact direct sur les hotes, mais peuvent egalement avoir des effets ecosystemiques plus larges en influencant le comportement des hotes. L'exploitation commerciale peut donc avoir d'importantes consequences ecologiques au-dela des effets directs de la reduction de la biomasse. Les coques (Austrovenus stutchburyi) sont souvent infectees par des parasites trematodes (Acanthoparyphium spp. et Curtuteria australis) et font l'objet d'une exploitation commerciale a Otago (Nouvelle- Zelande) depuis 1983. Afin d'evaluer les effets de l'exploitation sur l'abondance de trematodes chez A. stutchburyi, les nombres de parasites par coque dans des zones exploitees et non exploitees ont ete compares. Le nombre moyen de parasites par coque etait 36 % plus eleve aux sites exploites commercialement, donnant a penser que l'exploitation accroit la transmission locale des parasites. Il pourrait etre possible pour les exploitants de reduire leur influence sur les niveaux d'infection parasitaire dans les populations de coques en exploitant ces dernieres moins intensivement et en laissant s'ecouler plus de temps entre les recoltes pour permettre la regeneration de la biomasse de coques. [Traduit par la Redaction], Introduction Commercial harvesting reduces the population density of target species (Ludwig et al. 1993; Botsford et al. 1997), which can affect parasite transmission (Wood et al. 2010) and the intensity [...]

Eutrophication of shallow coastal ecosystems often manifests as dense mats of opportunistic macroalgae, degraded underlying sediments and displacement of ecologically important primary producers including seagrass beds. Ecological thresholds of drivers that cause these shifts in ecosystem state are needed to manage eutrophication symptoms before tipping points are crossed. This study quantifies total nitrogen (TN) load thresholds for abrupt changes in ecologically important seagrass habitat and the appearance of nuisance macroalgae as a function of estimated catchment TN load to shallow, intertidal estuaries in New Zealand. Results indicate that the extents of both macrophyte types are strongly governed by increasing TN load to the estuaries, while total suspended sediment load was not significant alone or as an interaction term. Seagrass extent was unpredictable below a critical TN load envelope (< 50 mg N m−2 d−1), likely because there are additional factors than TN that influence seagrass cover, but once surpassed, seagrass became restricted to less than 10% of intertidal area or absent. Conversely, nuisance macroalgae was predominantly absent unless TN load exceeded 41.15 ± 21.1 mg N m−2 d−1, expanding in extent by 2.7% of intertidal area for every 50 mg N m−2 d−1 increase above threshold levels. This research underscores the importance of managing diffuse nitrogen inputs to safeguard the structure and function of shallow estuarine ecosystems before tipping points in dominant macrophyte type are crossed. [ABSTRACT FROM AUTHOR]