Modelling the biogeochemical footprint of rivers in the Hauraki Gulf, New Zealand

Building accurate physical-biogeochemical models of processes driving climate and eutrophication-related stressors in coastal waters is an essential step in managing the impacts of these stressors. Here we develop a coupled physical-biogeochemical model to investigate present day processes for a key marine ecosystem in Aotearoa, New Zealand: The Hauraki Gulf/Firth of Thames system. Simulation results compared well with an accompanying long-term (decadal) observational dataset, indicating that the model captured most of the physical and biological dynamics of the Hauraki Gulf/Firth of Thames system. This model was used to investigate the riverine and cross shelf exchanges of nutrients in the region and showed that only a small number of large rivers within the Firth of Thames dominated the freshwater inputs, with phytoplankton concentrations driven by nutrient inputs from these rivers. However, while riverine inputs dominated the biological response in the Firth of Thames, cross-shelf fluxes dominated the biological response in the outer Hauraki Gulf region. Nutrients from both sources were balanced by a sediment denitrification flux. Analyses were conducted to examine agreement of observations with subsampled and climatological model outputs. These revealed that modelling effort needs to focus on the representation of sediment fluxes and parameterizations during the autumn, and the observational effort needs to focus on increased temporal data collection during summer to better understand biases in seasonal climatologies derived from model and observations. These results are valuable for demonstrating effects of land-derived and oceanic drivers of the biogeochemical dynamics of the Hauraki Gulf/Firth of Thames system

Vertical stratification of phytoplankton biomass in a deep estuary site: implications for satellite-based net primary productivity

The accuracy of satellite estimates for water column net primary productivity (NPP) are contingent upon the reliability of surface phytoplankton biomass, specifically chlorophyll a (Chl.a) and carbon (Cphyt), as indicators of euphotic biomass and photosynthetic rate. We assessed patterns in water column biomass at a deep estuary site (~40 m) in the Firth of Thames, Hauraki Gulf, New Zealand, using ten years (2005-2015) of in situ sampling (40 seasonal voyages and moored instrumentation). Seasonal biomass stratification coincided with physical and chemical stratification and exhibited a reasonable predictability based on surface Chl.a measures from mooring timeseries. High Chl.a (but not Cphyt) accumulated from late-spring (Nov.) in the lower portion of the water column, under nutrient deficient, clear surface water with deep euphotic zone conditions, peaking in mid-summer (Jan.) and ending by early autumn (Mar.). Satellite (MODIS-Aqua) NPP (2002-2018), was estimated with and without correction for deep biomass in two vertically generalized production models (Chl.a-VGPM and Cphyt-CbPM). Mean annual NPP (220-161 g C m-2 y-1, VGPM and CbPM respectively) increased 5-18% after accounting for euphotic zone deep biomass with a mid-summer maxim (Jan.: 30-33%). Interannual anomalies in biomass and NPP (about -10% to 10%) were an order of magnitude greater than small decreasing trends (<< 1% y-1). We discuss the impacts of observational factors on biomass and NPP estimation. We offer contextual insights into seasonal patterns by considering previous observations of biomass trends and nutrient enrichment in the Firth of Thames region. We propose future directions in accounting for deep biomass variations from shallow coastal areas to deeper continental shelf waters