Plankton Respiration, Production, and Trophic State in Tropical Coastal and Shelf Waters Adjacent to Northern Australia

Christian Lønborg,Murray Logan,Samantha Duggan,A. David McKinnon

doi:10.3389/fmars.2017.00346

Christian Lønborg, Murray Logan + Show 2 more

Open Access

https://doi.org/10.3389/fmars.2017.00346

Copy DOI

Abstract

In a changing ocean, tropical waters can be instructive as to the potential effects of climate induced changes on marine ecosystem structure and function. We describe the relationships between planktonic community respiration (CR), net community production (NCP), gross primary production (GPP) and environmental variables in 14 regions and three ecosystem types (coastal, coral reef and open sea) from Australia, Papua New Guinea and Indonesia. The data are compiled from separate studies conducted between 2002 and 2014 with the goal of better parameterizing the metabolic balance in tropical marine waters. Overall, these regions were strongly autotrophic (average GPP:CR ratio: 2.14 ± 0.98), though our dataset of 783 paired measurements did include some oceanic stations where heterotrophy (GPP:CR < 1) was predominant, and some coastal stations that were intermittently heterotrophic. Our statistical analysis suggested that temperature was the most important determinant of CR in coral reef and ocean ecosystems but less so in coastal ecosystems, where chlorophyll concentration was more important. In contrast, chlorophyll and sampling depth were more important in regulating GPP than temperature. The relationships between temperatures and metabolic rates showed that these were ecosystem-dependent, with coastal ecosystems showing less response to temperature than coral reef and open sea sites. The threshold of GPP to achieve metabolic balance fell in a range between 0.715 mmol O2 m-3 d-1 in the Coral Sea to 10.052 mmol O2 m-3 d-1 in mangrove waterways of Hinchinbrook Channel. These data allow regions in and around northern Australia to be ranked in terms of trophic state, ranging from the oligotrophic Scott Reef (GPP:CR = 0.84 ± 0.08) to productive surface waters of the Kimberley coast (GPP:CR = 5.21 ± 0.62). The measurement of pelagic metabolism shows potential as a quantitative tool to monitor the trophic state of coastal waters.

Highlights

Sixty years ago the concept of net community production (NCP), the difference between gross primary production (GPP), and the community respiration (CR) was framed by Odum (1956)
Our objective is three-fold: (1) to compare tropical ecosystems by partitioning our data into different ecosystem types; (2) to rank regions in order of metabolic activity, an approach we consider useful for understanding and managing coastal waters increasingly threatened by anthropogenic influences on ecosystem productivity mediated by environmental variables such as nutrients and light, and; (3) to identify factors influencing metabolism that have the potential to shift the trophic state of these ecosystems as a result of environmental changes
dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) concentrations of surface waters were close to the detection limits of standard methods, but occasionally higher levels were found at stations closer to shore directly affected by river runoff

Summary

Introduction

Sixty years ago the concept of net community production (NCP), the difference between gross primary production (GPP), and the community (autotrophic plus heterotrophic) respiration (CR) was framed by Odum (1956). While some studies suggest that these systems are net heterotrophic due to the delivery of large amounts of continentally derived organic matter (Smith and Mackenzie, 1987; Smith and Hollibaugh, 1993), others have found that globally the coastal ocean is autotrophic, exporting 15% of its NCP to the open ocean (Ducklow and McAllister, 2005). These differences could be due to differences in methodology or to spatio-temporal variation in metabolic balance. Spatio-temporal variation has been attributed to factors such as (1) changes in environmental conditions (e.g., temperature: Regaudie-de-Gioux and Duarte, 2012); (2) differences in the input and composition of nutrients (MartínezGarcía et al, 2010); (3) changes in plankton abundance and productivity of primary producers (Gasol and Duarte, 2000); (4) variation in the phytoplankton size structure and growth rate (Marañón, 2015); and (5) the amount and reactivity of the locally produced (autochthonous) and imported (allochthonous) organic matter (e.g., Serret et al, 1999; Robinson et al, 2002; Lønborg and Álvarez-Salgado, 2012)

Objectives

Methods

Results

Conclusion