Attributing Controlling Factors of Acidification and Hypoxia in a Deep, Nutrient-Enriched Estuarine Embayment

Abstract

Measuring and attributing controlling factors of acidification and hypoxia are essential for management of coastal ecosystems affected by those stressors. We address this using surveys in the Firth of Thames, a deep, seasonally stratified estuarine embayment adjoing the Hauraki Gulf in northern Aotearoa/New Zealand. The Firth’s catchment has undergone historic land-use intensification transforming it from native forest cover to dominance by pastoral use, increasing its riverine total nitrogen loading by ∼82% over natural levels and switching it’s predominate loading source from offshore to the catchment. We hypothesised that seasonal variation in net ecosystem metabolism [NEM: dissolved inorganic carbon (DIC) uptake/release] will be a primary factor determining carbonate and oxic responses in the Firth, and that organic matter involved in the metabolism will originate primarily by fixation within the Firth system and be driven by catchment dissolved inorganic nitrogen (DIN) loading. Seasonal ship-based and biophysical mooring surveys across the Hauraki Gulf and Firth showed depressed pH and O2 reaching pH ∼7.8 and O2 ∼4.8 mg L–1 in autumn in the inner Firth, matched by shoreward increasing nutrient loading, phytoplankton, organic matter, gross primary production (GPP) and apparent O2 utilization. A carbonate system deconvolution of the ship survey data, combined with other ship survey and mooring results, showed how CO2 partial pressure responded to seasonal shifts in temperature, NEM, phytoplankton sinking and mineralisation and water column stratification, that underlay the late-season expression of acidification and hypoxia. This aligned with seasonal shifts in net DIC fluxes determined in a coincident nutrient mass-balance analysis, showing near-neutral fluxes from spring to summer, but respiratory NEM from summer to autumn. Particulate C:N and ratios of organic C fixed by Firth GPP to that from river inputs (∼29- to 100-fold in summer and autumn) showed that the dominant source of organic matter fuelling heterotrophy in autumn was autochthonous GPP, driven by riverine DIN loading. The results signified the sensitivity of deep, long-residence time, seasonally stratifying estuaries to acidification and hypoxia, and are important for coastal resource management, including aquaculture developments and catchment runoff limit-setting for maintenance of ecosystem health.

Microplankton Interactions with Decadal-Scale Nutrient Enrichment in a Deep Estuary, with Implications for Eutrophication—Related Ecosystem Stressors

“…This association with DIN was consistent with findings in coastal ecosystems showing that biomass is demonstrably related to nutrient loading rates and concentrations (Boynton and Kemp 2008;Boynton et al 1982;Monbet 1992), albeit with variability in the relationships across different types of coastal ecosystems (Cloern and Jassby 2008). The long residence times and relatively clear waters of large coastal embayments such as the Firth mean such systems are sensitive to nutrient enrichment (Zeldis et al 2022). They are susceptible because, in addition to having potentially high nutrient loading from large catchments, they can support phytoplankton blooms which undergo complete cycles of growth, consumption and senescence, prior to seaward export (Ferreira et al 2005).…”

“…Over the time series, DON increased most strongly in autumn, potentially from mineralisation of organic matter in summer and autumn (Fig. 3; Zeldis et al (2022)). This mineralisation would be driven by bacteria, which increased strongly in summer, over the time series.…”

“…Additional seasonal surveys were conducted on a station transect through the Firth and into the Hauraki Gulf (Fig. 1B) in spring, summer, autumn and winter 2012-2013 and autumn 2010, as part of research in the wider Hauraki Gulf/Firth region that examined its carbonate and O 2 environments (Zeldis et al 2022). These surveys used CTDrosette sampling and analysis protocols identical to those at the Firth monitoring site.…”

“…Currently, about 78% of TN runoff to the Firth is anthropogenic (Vant 2013), and catchment runoff now dominates its nutrient loading, with ~ 85% of its dissolved inorganic nitrogen (DIN) supply arising from its catchment (Zeldis and Swaney 2018). As a consequence, the Firth now sustains volumetric primary production roughly double that of the seaward Hauraki Gulf (Gall and Zeldis 2011;Zeldis et al 2022;Zeldis and Willis 2015). It has a pronounced seasonal cycle of net ecosystem metabolism that evolves from net autotrophy in spring and early summer to net heterotrophy from late summer through autumn, when it often expresses significantly depressed pH and hypoxia (Law et al 2019;Zeldis et al 2022;Zeldis and Swaney 2018).…”

“…As a consequence, the Firth now sustains volumetric primary production roughly double that of the seaward Hauraki Gulf (Gall and Zeldis 2011;Zeldis et al 2022;Zeldis and Willis 2015). It has a pronounced seasonal cycle of net ecosystem metabolism that evolves from net autotrophy in spring and early summer to net heterotrophy from late summer through autumn, when it often expresses significantly depressed pH and hypoxia (Law et al 2019;Zeldis et al 2022;Zeldis and Swaney 2018).…”