Effects of changes in nutrient loading and composition on hypoxia dynamics and internal nutrient cycling of a stratified coastal lagoon

Yafei Zhu,Perran L M Cook,Andrew Mccowan

doi:10.5194/bg-14-4423-2017

Yafei Zhu, Perran L M Cook + Show 1 more

Open Access

https://doi.org/10.5194/bg-14-4423-2017

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Abstract

Abstract. The effects of changes in catchment nutrient loading and composition on the phytoplankton dynamics, development of hypoxia and internal nutrient dynamics in a stratified coastal lagoon system (the Gippsland Lakes) were investigated using a 3-D coupled hydrodynamic biogeochemical water quality model. The study showed that primary production was equally sensitive to changed dissolved inorganic and particulate organic nitrogen loads, highlighting the need for a better understanding of particulate organic matter bioavailability. Stratification and sediment carbon enrichment were the main drivers for the hypoxia and subsequent sediment phosphorus release in Lake King. High primary production stimulated by large nitrogen loading brought on by a winter flood contributed almost all the sediment carbon deposition (as opposed to catchment loads), which was ultimately responsible for summer bottom-water hypoxia. Interestingly, internal recycling of phosphorus was more sensitive to changed nitrogen loads than total phosphorus loads, highlighting the potential importance of nitrogen loads exerting a control over systems that become phosphorus limited (such as during summer nitrogen-fixing blooms of cyanobacteria). Therefore, the current study highlighted the need to reduce both total nitrogen and total phosphorus for water quality improvement in estuarine systems.

Highlights

Excessive anthropogenic nutrient loading, nitrogen, has led to widespread hypoxia and other ecological damages in estuarine and coastal areas (Howarth et al, 2011)
The annual total primary production (TPP) rate in Lake Wellington could reach as high as 600 g C m2 year−1, about 350 g C m2 year−1 in Lake Victoria followed by 250 g C m2 year−1 in Lake King (Fig. 3)
TPP was more sensitive to reductions in total nitrogen (TN) than total phosphorus (TP) until reduction exceeded 75 % when TPP became more sensitive to TP

Summary

Introduction

Nitrogen, has led to widespread hypoxia and other ecological damages in estuarine and coastal areas (Howarth et al, 2011). High primary production as a result of eutrophication can lead to hypoxia or anoxia in poorly mixed bottom water and subsequently enhance the recycling of both nitrogen and phosphorus, which again can reinforce eutrophication (Correll, 1998). This has been found in many stratified estuarine systems around the world, including the Baltic Sea (Vahtera et al, 2007) and the Black Sea (Capet et al, 2016), the Neuse River Estuary (Paerl et al, 1995), and the Gippsland Lakes (Scicluna et al, 2015). Conley et al (2002) found that the annual change in dissolved inorganic phosphorus (DIP) in the Baltic Sea was proportional to the area covered by hypoxic water rather than the catchment phosphorus load

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