Patterns of Ecosystem Metabolism in the Tonle Sap Lake, Cambodia with Links to Capture Fisheries

Gordon W Holtgrieve,Dirk Lamberts,Matti Kummu,Jorma Koponen,Mauricio E Arias,Eric J Ward,Jeffrey E Richey,Kim N Irvine,Juha Sarkkula,Z Daniel Deng

doi:10.1371/journal.pone.0071395

Abstract

The Tonle Sap Lake in Cambodia is a dynamic flood-pulsed ecosystem that annually increases its surface area from roughly 2,500 km2 to over 12,500 km2 driven by seasonal flooding from the Mekong River. This flooding is thought to structure many of the critical ecological processes, including aquatic primary and secondary productivity. The lake also has a large fishery that supports the livelihoods of nearly 2 million people. We used a state-space oxygen mass balance model and continuous dissolved oxygen measurements from four locations to provide the first estimates of gross primary productivity (GPP) and ecosystem respiration (ER) for the Tonle Sap. GPP averaged 4.1±2.3 g O2 m−3 d−1 with minimal differences among sites. There was a negative correlation between monthly GPP and lake level (r = 0.45) and positive correlation with turbidity (r = 0.65). ER averaged 24.9±20.0 g O2 m−3 d−1 but had greater than six-fold variation among sites and minimal seasonal change. Repeated hypoxia was observed at most sampling sites along with persistent net heterotrophy (GPP<ER), indicating significant bacterial metabolism of organic matter that is likely incorporated into the larger food web. Using our measurements of GPP, we calibrated a hydrodynamic-productivity model and predicted aquatic net primary production (aNPP) of 2.0±0.2 g C m−2 d−1 (2.4±0.2 million tonnes C y−1). Considering a range of plausible values for the total fisheries catch, we estimate that fisheries harvest is an equivalent of 7–69% of total aNPP, which is substantially larger than global average for marine and freshwater systems. This is likely due to relatively efficient carbon transfer through the food web and support of fish production from terrestrial NPP. These analyses are an important first-step in quantifying the resource pathways that support this important ecosystem.

Highlights

The accumulation and processing of carbon (C) and energy within ecosystems has been a dominant theme in ecology for much of its history and is an important determinant of ecosystem functioning
We considered the same set of relationships using daily metabolism estimates and found weak but significant relationships between gross primary productivity (GPP) and turbidity (Kendall t = 0.27), ecosystem respiration (ER) and water temperature (t = 0.20), ER and average dissolved oxygen (DO) (t = -0.45)
The widespread and repeated hypoxia observed in the Tonle Sap lake and floodplain (Figure 2) is most likely a natural phenomenon due to a combination of high water temperatures leading to low oxygen solubility and organic matter inputs to the aquatic environment from both the Mekong

Summary

Introduction

The accumulation and processing of carbon (C) and energy within ecosystems has been a dominant theme in ecology for much of its history and is an important determinant of ecosystem functioning (sensu [1]). Studies of aquatic ecosystem metabolism – rates of gross primary productivity and ecosystem respiration – describe the overall magnitude and relative balance of heterotrophic and autotrophic processes within an ecosystem [5], and are extremely useful in understanding critical ecosystem processes such as energy flows, nutrient cycling, carbon balance, trophic state, and food web dynamics [6]. Floodplains are inherently dynamic and cannot be categorized as aquatic or terrestrial, but instead are defined by seasonal changes in the hydrologic environment. These ‘‘flood-pulse’’ ecosystems [7] alternate in their functioning between a primarily aquatic-phase and primarily terrestrial-phase, with each influencing the other (e.g., through sediment deposition or organic matter production)

Methods

Results

Discussion

Conclusion