Oxygen Consumption and Organic Matter Remineralization in Two

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Characterization of Natural and Affected Environments

Oxygen consumption and organic matter remineralization in two subtropical, eutrophic coastal embayments Hongjie Wang, Xinping Hu, Michael Wetz, and Kenneth C Hayes Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02971 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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Environmental Science & Technology

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Oxygen consumption and organic matter remineralization in two subtropical,

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eutrophic coastal embayments

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Hongjie Wang1, Xinping Hu1, Michael Wetz2, Kenneth Hayes2

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1. Department of Physical and Environmental Sciences, Texas A&M University – Corpus

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Christi

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2. Department of Life Sciences, Texas A&M University – Corpus Christi

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Manuscript prepared for Environmental Science & Technology

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Corresponding author:

Xinping Hu ([email protected], Tel: 361-825-3395)

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Keyword

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Dissolved Oxygen, Hypoxia, Stable Isotopes, Estuary, Lagoon, Eutrophication

ACS Paragon Plus Environment

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Abstract There is a strong need to understand sources of organic matter to coastal lagoons, as

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these systems often have long water residence times, are susceptible to eutrophication,

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and display symptoms such as low oxygen conditions. We found that the integrated

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dissolved oxygen (DO) consumption in the water column accounted for 67~73% of total

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DO consumption in two eutrophic coastal lagoons (Baffin Bay and Oso Bay) in the

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northwestern Gulf of Mexico. δ13C of particulate organic carbon (δ13CPOC) showed

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temporal variations that corresponded with hydrological condition changes in Baffin Bay

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but less temporal changes in Oso Bay, whereas the lower δ15NPON values in Baffin Bay

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indicated more of agricultural influence than in Oso Bay, where urban sewage influences

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dominated. Based on closed-system incubation experiments, water column respiration in

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Baffin Bay was driven by the respiration of a combination of phytoplankton, carbon from

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nearshore and benthic macrophytes, and other allochthonous organic carbon sources

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depending on hydrological conditions. However, respiration of algal carbon dominated

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DO consumption in Baffin Bay sediments. In comparison, Oso Bay water column

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respiration was largely attributed to the degradation of phytoplankton, the growth of

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which was sustained by nutrient discharge from wastewater treatment plants in the

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watershed. Contrasting to the water column, seagrass and saltmarsh carbon appeared to

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be the primary organic carbon source that drove DO consumption in Oso Bay sediments.

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These observations highlight the complexity of organic carbon sources that contribute to

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DO consumption in estuaries affected by human activities, especially in systems with

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long water residence times that can retain both organic matter and nutrients for extended

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periods of time.


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Table of Contents (TOC)/Abstract Art

Baffin Bay Water -10

Water column POC Water column DOC

-13

Remineralized OC

13C

-16 -19 -22 -25 -28 -31 10 38

15

20

25

30

35

40

45

Salinity


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1. Introduction

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Cultural eutrophication is a major environmental threat facing coastal ecosystems

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worldwide 1-3. Over the past ~50 years, nutrient loading to the coastal zone has

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substantially increased, resulting in symptoms such as persistent algal blooms and bottom

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water hypoxia (i.e., water dissolved oxygen or DO < 2 mg L-1) 1, 4, 5. Located in a semi-

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arid subtropical region, coastal embayments along the northwestern Gulf of Mexico

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(nwGoM) are affected by climate change (i.e., long-term water temperature increase),

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land use changes, increasing human freshwater demand, and eutrophication 6-8. One

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consequence of these large-scale changes is bottom water hypoxia, which has been

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observed in a number of estuaries in this region as far back as in 1988 6, 9-11. In some

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instances, hypoxic conditions have led to fish kills 9 (unpubl. Texas Parks & Wildlife fish

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kill report). Studies have also shown that hypoxia reduces benthic infauna abundance 10-

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, thus elucidating the mechanisms that drive hypoxia formation is important. In general, coastal hypoxia is driven by a combination of warm water temperature,

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high organic matter loading, and water column stratification 5, 13-15. However, despite the

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well-documented increase in nutrient and organic matter loading and concomitant

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increase in the prevalence of phytoplankton blooms in many coastal systems 1, 3, 5, 16, 17,

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the source(s) of organic matter that fuels microbial respiration-induced DO consumption

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is not always clear because of the diverse organic matter origins in these systems 18-20.

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Such complexity hinders the understanding of hypoxia formation mechanisms and thus

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prevents developing effective mitigation strategies.

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In many estuaries, organic matter derived from in situ production is believed to be

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the dominant carbon source that supports water column respiration. Consequently, efforts


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have been put forth to control inorganic nutrient loading5, 21-27. However, most coastal

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areas also receive significant allochthonous organic matter loading. Organic matter

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derived from municipal wastewater discharge, manure-based agricultural operations, and

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concentrated animal waste operations may be highly labile, and can be rapidly

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remineralized once it enters the coastal water bodies 18, 19, 28-30.

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In oceanographic and geochemical studies, stable nitrogen isotopic composition

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(δ15N) of organic matter is an indicator of trophic levels and thus can be used to show

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anthropogenic influences 31, 32. Meanwhile, stable carbon isotopic composition (δ13C) of

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organic matter is often used to identify carbon sources 33-36. Given the complexity of

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organic carbon sources and their differences in lability, an alternative approach to

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identify the oxygen-consuming reactant is to use δ13C of the respiration produced CO2

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(δ13Cacc) 25, 37-40.

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In this study, we measured water column and surface sediment DO consumption

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rates, determined δ13C of particulate organic carbon (δ13CPOC), dissolved organic carbon

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(δ13CDOC), remineralized organic carbon (δ13COCx), as well as δ15N of particulate organic

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nitrogen (δ15NPON) in two lagoonal estuaries that differ in watershed land use but both

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experience seasonal hypoxia 6, 7. Finally, we discussed the organic carbon sources that

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controlled DO consumption in these systems. This study represents an effort that meets

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the need of understanding the drivers of hypoxia in coastal lagoons, which typically have

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long water residence times and are highly susceptible to impacts from both human

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activities in the watershed (land use change and nutrient loading) and climate change41, 42.

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2. Method

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2.1 Study area


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We examined two south Texas lagoonal estuaries in the nwGoM, Baffin Bay and

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Oso Bay (Fig. 1). Both bays are undergoing eutrophication, as exemplified by a long-

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term increase in nutrient and chlorophyll a (Chl a) concentrations and incidence of

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episodic hypoxia 6, 7, 43. In addition, Baffin Bay has also experienced prolonged (multi-

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year), dense blooms of the “brown tide” alga, Aureoumbra lagunensis 6, 44. Water

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residence time in Baffin Bay is about one year and freshwater input is on average less

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than evaporation, thus the water is often hypersaline 45. Oso Bay is a sub-embayment of

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Corpus Christi Bay to the north and is influenced by rapid urbanization, and its

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freshwater predominantly comes from wastewater treatment plants (a total of three) 7.

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The average water depths in Baffin Bay and Oso Bay are ~ 2 m and 1 m, respectively.

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2.2 Field sampling

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Water samples were collected at each station using a horizontal Van Dorn sampler

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and dispensed into 1-L precombusted borosilicate glass bottles. Near surface water

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(sampling depth 20-30 cm) was collected monthly (during 11/2014 - 09/2016 at nine

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stations in Baffin Bay and during 08/2014 - 09/2016 at six stations in Oso Bay, Fig. 1) for

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DO, Chl a concentration, stable isotope analysis of particulate and dissolved organic

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matter. Both near surface water and bottom water (within 20-30 cm from the sediment-

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water interface) for oxygen consumption incubations were also collected in Baffin Bay

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(stations BB3 and BB6). Only surface water at Station Oso Ward was collected given the

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shallow water depth (~1 m) (Fig. 1). Duplicate sediment cores (~20 cm) were collected at

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these three stations for the sediment DO consumption incubation using a custom-made

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corer following the design of Gardner and McCarthy 46.

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2.3 Oxygen consumption incubation


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Due to logistical constraints, fewer surface water incubations were carried out than

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those using bottom waters for Baffin Bay. DO consumption rate (Kwater, unit in mmol O2

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m-3 day-1) was measured in duplicate at in situ temperature in the dark for 24 hours using

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250 mL pre-combusted BOD bottles under magnetic stirring. DO in the BOD bottle was

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continuously monitored and recorded using calibrated YSITM ProODO optical oxygen

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sensors every two minutes (Fig. S1). Assuming that the average water depths were 2 m in

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Baffin Bay and 1 m in Oso Bay and that the water columns were well mixed, we

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calculated the integrated water column DO consumption rates (mmol m-2 day-1) by

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multiplying the Kwater in bottom water by the average water depths.

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After all sediment cores were transported to the lab, all cores were allowed to settle

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for at least 4 hours, then the overlying water was aerated using an air pump (about 30

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mins) to make sure that DO saturation was close to 100% prior to incubations. Each core

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tube was sealed at the top using a plastic plate without headspace (Fig. S1), which was

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equipped with a motor that drove a suspended magnetic stir bar underneath and a port

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that allowed insertion of a ProODO optical oxygen sensor. The cores were incubated at in

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situ temperature in a circulating water bath. In both water and sediment incubations, DO

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concentration decreased linearly with time, thus a simple linear regression was used to

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calculate DO consumption rate 47. DO consumption rate at the sediment-water interface

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was calculated based on the difference between the total DO consumption rate (Ktotal,

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mmol O2 m-3 day-1) during sediment core incubation and that from water incubation only

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(Kwater, mmol O2 m-3 day-1) following Eq. 1 (h is the thickness of the overlying water in

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the incubating cores).

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,

= (K

−K

)×h


(1)

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An example of how the water and sediment DO consumption rate was calculated is

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included in the Supplementary Information (Fig. S2, Table S1). Note that we did not

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directly measure Kwater from November 2014 to June 2015 but used the averaged Kwater

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for each bay during the entire study period for these months.

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2.4 Organic carbon source incubation

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Seasonal organic carbon source incubations from December 2014 to August 2016

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were carried out using both water and sediment samples from stations BB3, BB6, and

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Oso Ward. Briefly, the water was distributed into seven 40 ml FisherbrandTM borosilicate

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glass EPA vials with Teflon-lined silicone septa without headspace. The vials were

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incubated in the dark at room temperature (~22°C). Every other day, one vial was taken

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to arrest the microbial process by injecting 100 μL saturated HgCl2, then the water

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samples for stable isotope analysis (i.e., δ13C of dissolved inorganic carbon, or DIC) were

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transferred into separate storage vessels. The δ13CDIC samples were stored in 2-ml

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borosilicate autosampler vials (Restek®) with natural rubber septa lined open-top

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aluminum caps. For sediment incubations, the surface 1-2 cm sediment was first

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homogenized with bottom water, and then the sediment slurry was filled into seven 40 ml

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EPA vials. One vial was sacrificed per day for water extraction, and the overlying water

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was filtered through 0.45 μm sterile nylon disc filters into separate vials for DIC, δ13CDIC,

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and calcium analyses. The DIC and Ca2+ samples were stored in 4-ml borosilicate vials,

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which were tightly closed using phenolic screw caps with PTFE-faced rubber liner. All

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samples were stored at 4°C until analysis, typically within 2 months of collection.

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The δ13CDIC in the incubation samples were linearly related to the reciprocal of the DIC concentration ([DIC]-1), with the intercept of the regression representing the δ13C of


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the accumulated DIC (δ13Cacc) during the incubation process 38, 39, 48. In sediment

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incubations, δ13Cacc signal may include contributions from both the remineralized organic

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carbon (δ13COCx) and carbonate dissolution (δ13CIC) (Eq. 2). Assuming all dissolved

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carbonate was CaCO3, then the isotope mass balance in sediment incubations is:

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δ C

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The DIC increase due to carbonate dissolution was assumed to be the same as the

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increase in calcium concentration ([Ca2+]) during the incubation. Then, fIC was calculated

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as ratio of [Ca2+] increase and DIC concentration increase.

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2.5 Analytical methods

= (1 − f ) × δ C

+f ×δ C

(2)

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Field temperature, salinity, and DO were recorded using a pre-calibrated YSITM

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Professional Plus multisonde. Chl a samples were collected and analyzed following the

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protocol in Wetz et al. 7. DIC concentration was measured using infrared detection

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following acid extraction on an Apollo® AS-C3 DIC analyzer with the analytical

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precision of better than 0.1%. Certified reference material from Dr. A. Dickson's lab

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(Scripps Institution of Oceanography) was used to calibrate the DIC analyzer. [Ca2+] was

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determined using a Metrhom® automatic titrator in conjunction with a Ca2+ ion-selective

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electrode for endpoint detection. The analytical precision was 0.2% 39. δ13CDIC was

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determined on a Thermo Fisher Delta VTM isotope ratio mass spectrometer (IRMS) with a

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GasBench II preparation module for trace gas samples, and the analytical precision was

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±0.1‰. Samples for δ13CPOC and δ15NPON analyses were collected by filtering water

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samples through precombusted (at 450°C) 47-mm Whatman® GF/F filters. The filters

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were then acidified using HCl fume for 4 hours to remove carbonate minerals. The

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decarbonated filters were dried overnight at 65°C, and then were analyzed on a Thermo


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FlashEA® 1112 elemental analyzer coupled IRMS 49. Isotopic values were calibrated

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using chitin standards (Sigma Chemical), which were calibrated to a NIST stable isotope

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(USGS 40) standard. The precision was ±0.1‰. The δ13CDOC of selected filtrates from the

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previous step was obtained using the combination of an Aurora 1030C high temperature

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catalytic conversion DOC analyzer, a molecular sieve trap and a continuous flow IRMS.

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USGS 61 caffeine and USGS 62 caffeine were used as standards in the DOC analysis.

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The precision of δ13CDOC analysis was ±0.2‰ 50.

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3. Results

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3.1. Temporal variations of salinity and temperature

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Water temperature reflected clear seasonal patterns in both Baffin Bay and Oso Bay

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(Fig. 2a). Higher temperatures (23.1°C - 30.2°C) were observed from April to October,

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and lower temperature (12.8°C - 20.5°C), from November to March. Salinity in Baffin

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Bay (Fig. 2b) was the highest (54.5) in August 2014. Afterwards, it decreased sharply

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from February 2015 to June 2015, due to a series of strong flooding events. Salinity

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increased from June to September, and stayed in a relatively narrow range (34.6±4.6)

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from October 2015 to November 2016. In comparison, salinity in Oso Bay was generally

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lower than that in Baffin Bay.

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DO displayed a clear seasonal pattern that can be linked to temperature in both

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Baffin Bay and Oso Bay, with lowest levels observed in the warmer months and highest

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levels in cooler months (Fig. 2c&d). Relatively low oxygen (27°C, 48.6±18.3 mmol O2 m-2 day-1) than under cooler condition (

Oxygen Consumption and Organic Matter Remineralization in Two

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