Lagoon Biogeochemical Processing is Reflected in Spatial Patterns of Sediment Stable Isotopic Ratios

Elizabeth Burke Watson,Juan Carlos Herguera,Julianna Mcdonnell,Andrew B Gray,Johannes R Krause,Michelle E Gannon,Karen Raquel Villegas Villegas Manríquez,Alejandro Hinojosa-Corona

doi:10.3390/jmse8110874

Abstract

The spatial analysis of biota, particulate organic matter, and sediments for stable isotopes of carbon (δ13C), nitrogen (δ15N), and sulfur (δ34S) have proved useful for identifying patterns in productivity, nutrient pollution, and relationships between biological and physiochemical variables at the local and global scales. Yet such approaches are rarely applied to studies of lagoon or estuarine metabolism. Focusing on Bahía San Quintín, a heterotrophic seagrass-dominated lagoon on the Pacific coast of Baja California, México, we report on spatial patterns in surficial sediment CNS stable isotopic ratios as tracers of lagoon biogeochemical function. Stable nitrogen isotopes highlighted potential spatial variability in the balance between denitrification and nitrogen-fixation within the lagoon and identified an association between elevated δ15N levels and oyster culture, suggesting that oyster presence may be enhancing N2 production. Spatial patterns in δ34S covaried with sediment particle size, underlining the importance of sediment texture in determining the depth of sub-oxic-anoxic redox zones. Sediment carbon stable isotope ratios highlighted the lack of incorporation of seagrass carbon into seagrass meadow sediments, thus emphasizing the importance of phytoplankton or microphytobenthos for carbon accumulation in seagrass meadows. This report highlights the value of sediment isotopic values in corroborating spatial patterns in estuarine metabolism or macronutrient processing identified from chamber or flux-based studies. Stable isotope mapping can provide a useful addition to assessment of estuarine metabolism, or act as a stand-alone tool for generating hypotheses, identifying the influence of spatial gradients, and/or suggesting prime locations for investigation of microbial abundance or function.

Highlights

In recent years, the analysis of biota, particulate organic matter, and sediments for stable isotopes of carbon (C), nitrogen (N), and sulfur (S) has emerged as a common and powerful approach for assessing estuarine, nearshore, and open-ocean ecosystems [1,2]
Despite the demonstrated ability of landscape and regional-scale stable isotope analysis to uniquely reveal spatial gradients in ocean productivity [6], nutrient pollution plumes [7], and nutrient recycling [8], studies have only rarely applied this approach to coastal estuaries and lagoons [9]
Sediment δ15N can be considered a rough indicator of the balance between denitrification, which enriches the 15N pool through loss of 14N to the atmosphere, and N-fixation, which introduces light N isotopic signatures (0% ) [78], our results indicate N-fixation is more common in the upper arms of the lagoon, whereas denitrification is more common near the mouth where oceanic imports of nitrate occur

Summary

Introduction

The analysis of biota, particulate organic matter, and sediments for stable isotopes of carbon (C), nitrogen (N), and sulfur (S) has emerged as a common and powerful approach for assessing estuarine, nearshore, and open-ocean ecosystems [1,2]. Despite the demonstrated ability of landscape and regional-scale stable isotope analysis to uniquely reveal spatial gradients in ocean productivity [6], nutrient pollution plumes [7], and nutrient recycling [8], studies have only rarely applied this approach to coastal estuaries and lagoons [9]. We focus on the ability of spatial patterns in CNS sediment isotope ratios (δ15N, δ34S, δ13C) to provide insight into patterns of macronutrient processing across a lagoonal landscape, including potential effects of seagrass presence and oyster aquaculture on biogeochemical cycling. The balance of denitrification vs. N-fixation in most coastal lagoons remains unresolved, stable N isotopes can be useful for discriminating N-fixation (which introduces bioavailable N at isotopic values ≈ 0% ), and denitrification which consumes lighter N at a faster rate, leaving the NO3 pool substantially enriched in 15N [5]

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Results

Conclusion