DOI: 10.5061/dryad.8sf7m0cj0
title: Understanding the fate of shrimp aquaculture effluent in a mangrove ecosystem: aiding management for coastal conservation

description: 1. Areas dedicated to shrimp aquaculture have increased dramatically over
 the last 50 years. Resultant land-use changes directly threaten the extent
 of mangroves and yield conflicts on the discharge location of aquaculture
 effluent. 2. Khung Krabaen Bay (KBB), Thailand, is reforesting mangroves
 while increasing the efficiency of shrimp aquaculture for local farmers.
 In this coupled shrimp farm-mangrove system, effective management requires
 understanding the fate of aquaculture organic matter (OM) in the coastal
 environment. 3. We examined carbon and nitrogen stable isotope ratios
 (δ13C, δ15N) in primary producers and pools of particulate and sediment OM
 (POM, SOM) from the KKB mangrove and marine ecosystem to determine how
 shrimp aquaculture OM contributes to the coastal environment. Here,
 soy-based shrimp feed resulted in low shrimp δ15N, similar to marine POM,
 and thus we focus on the use of δ13C in tracking shrimp pond effluent in
 the environment. 4. δ13C signatures of SOM varied significantly along a
 land-to-ocean gradient (–29.1 to –23.9‰). We found consistently depleted
 mangrove SOM δ13C signatures (–29.4 to –28.2‰) indicating mangrove leaf
 litter is the primary source of OM to mangrove sediments, and there is
 little evidence that marine and shrimp pond OM contributes to the mangrove
 habitat. In contrast, relatively low δ13C values for marine SOM (–25.7 to
 –23.9‰) overlap with the δ13C of shrimp feed (–25.3‰) and Bayesian mixing
 models indicate that shrimp aquaculture feed and mangrove vegetation
 contribute the greatest OM to the marine ecosystem. 5. Compared to 20
 years ago, marine SOM δ13C signatures are depleted by ~10‰ and similar
 throughout KKB, indicating a homogenization of marine SOM carbon sources
 from 1998 to 2018. 6. Synthesis and applications. The doubling of shrimp
 aquaculture in KKB since 1998 led to enhanced discharge to the bay,
 swamping OM contributions from 13C-enriched seagrasses and marine
 plankton. Because of this increase in effluent release to KKB, the marine
 ecosystem is likely being more chemically impacted than the mangrove and
 should also be a focus of conservation efforts. Continued technological
 improvements (e.g., closed-systems, better feed efficiency) and support to
 local aquaculture farmers will help reduce OM discharge to coastal
 ecosystems and promote sustainable farming practices.06-Jan-2020
description: Please see the paper Hargan
 et al. in Journal of Applied Ecology for a description of the collection
 methods and more details on sample preparation.  Dried bulk sediment subsamples
 from the top 1 cm of surface sediment were sieved (to remove shells,
 rootlets, and stones), ground, and ~30.0 mg was weighed and folded into
 9x10 mm tin capsules for %N and δ<sup>15</sup>N. Acidified
 sediment samples (~12.0 - 20.0 mg) were weighed into separate 9 x 5 mm tin
 capsules for %C<sub> </sub>and
 δ<sup>13</sup>C<sub> </sub>analysis. Approximately
 5.0 mg of ground mangrove and seagrass leave sample was utilized for
 stable isotope analysis. The
 <sup>13</sup>C:<sup>12</sup>C and
 <sup>15</sup>N:<sup>14</sup>N ratios were analyzed
 by continuous-flow isotope-ratio mass spectrometry. The spectrometer
 (ThemoFisher Delta V Plus stable-isotope analyzer coupled with a Flash
 Elemental Analyzer) was operated in dual isotope mode at The David W. and
 Claire B. Oxtoby Environmental Isotope Lab (Pomona College,
 California). Analytical precision was
 determined using Acetanilide for δ<sup>13</sup>C (–29.53‰) and
 δ<sup>15</sup>N (1.18‰) (Indiana University) and %C (41.86%)
 and %N (16.18%) (ThermoFisher Scientific). For both
 δ<sup>13</sup>C and<i>
 </i>δ<sup>15</sup>N, the standard deviation of
 acetanilide on 10 replicate samples was &lt; 0.2‰<i>.
 </i>Samples were calibrated to the standards USGS40 and USGS24 for
 <sup>13</sup>C, and USGS40 and IAEA N-2 for
 <sup>15</sup>N. Results of stable isotope analyses are
 reported in δ notation where δ =
 [(R<sub>sample</sub>/R<sub>standard</sub>) – 1] x
 1000. R<sub>sample</sub> are the ratios of the isotopes (i.e.
 <sup>13</sup>C/<sup>12</sup>C,
 <sup>15</sup>N/<sup>14</sup>N) in samples, and
 R<sub>standard</sub> are the ratios of isotopes in Vienna Pee
 Dee Belemnite for δ<sup>13</sup>C and atmospheric nitrogen for
 δ<sup>15</sup>N.