Heterogeneous dissolved organic nitrogen supply over a coral reef: first evidence from nitrogen stable isotope ratios

Thibodeau, Benoît; Miyajima, Toshihiro; Tayasu, Ichiro; Wyatt, Alex S. J.; Watanabe, Atsushi; Morimoto, Naoko; Yoshimizu, Chikage; Nagata, Toshi

doi:10.1007/s00338-013-1070-9

Cited by 17 publications

(12 citation statements)

References 36 publications

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“…Furthermore, it is well-known that cyanobacterial mats and turf algae on coral reefs fix N 2 and that δ 15 N signatures of cyanobacterial mats are lower than of other benthic primary producers ( Table 3 ; Kayanne et al, 2005 ; Kolasinski et al, 2016 ). Released products from these mats contain N compounds lower in δ 15 N than N released by macroalgae or pelagic primary producers ( Kayanne et al, 2005 ; Thibodeau et al, 2013 ; Brocke et al, 2015 ). In the Caribbean significant increases in cyanobacterial mat cover on coral reefs were reported from less than 10 to more than 20% since ∼1990 ( De Bakker et al, 2017 ).…”

Section: Discussionmentioning

confidence: 99%

Spatio–temporal variation in stable isotope signatures (δ¹³C and δ¹⁵N) of sponges on the Saba Bank

Duyl

Mueller

Meesters³

2018

PeerJ

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Sponges are ubiquitous on coral reefs, mostly long lived and therefore adaptive to changing environmental conditions. They feed on organic matter withdrawn from the passing water and they may harbor microorganisms (endosymbionts), which contribute to their nutrition. Their diets and stable isotope (SI) fractionation determine the SI signature of the sponge holobiont. Little is known of spatio–temporal variations in SI signatures of δ13C and δ15N in tropical sponges and whether they reflect variations in the environment. We investigated the SI signatures of seven common sponge species with different functional traits and their potential food sources between 15 and 32 m depth along the S-SE and E-NE side of the Saba Bank, Eastern Caribbean, in October 2011 and October 2013. SI signatures differed significantly between most sponge species, both in mean values and in variation, indicating different food preferences and/or fractionation, inferring sponge species-specific isotopic niche spaces. In 2011, all sponge species at the S-SE side were enriched in d13C compared to the E-NE side. In 2013, SI signatures of sponges did not differ between the two sides and were overall lighter in δ13C and δ15N than in 2011. Observed spatio–temporal changes in SI in sponges could not be attributed to changes in the SI signatures of their potential food sources, which remained stable with different SI signatures of pelagic (particulate organic matter (POM): δ13C −24.9‰, δ15N +4.3‰) and benthic-derived food (macroalgae: δ13C −15.4‰, δ15N +0.8‰). Enriched δ13C signatures in sponges at the S-SE side in 2011 are proposed to be attributed to predominantly feeding on benthic-derived C. This interpretation was supported by significant differences in water mass constituents between sides in October 2011. Elevated NO3 and dissolved organic matter concentrations point toward a stronger reef signal in reef overlying water at the S-SE than N-NE side of the Bank in 2011. The depletions of δ13C and δ15N in sponges in October 2013 compared to October 2011 concurred with significantly elevated POM concentrations. The contemporaneous decrease in δ15N suggests that sponges obtain their N mostly from benthic-derived food with a lower δ15N than pelagic food. Average proportional feeding on available sources varied between sponge species and ranged from 20% to 50% for benthic and 50% to 80% for pelagic-derived food, assuming trophic enrichment factors of 0.5‰ ± sd 0.5 for δ13C and 3‰ ± sd 0.5 for δ15N for sponges. We suggest that observed variation of SI in sponges between sides and years were the result of shifts in the proportion of ingested benthic- and pelagic-derived organic matter driven by environmental changes. We show that sponge SI signatures reflect environmental variability in space and time on the Saba Bank and that SI of sponges irrespective of their species-specific traits move in a similar direction in response to these environmental changes.

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Section: Discussionmentioning

confidence: 99%

Spatio–temporal variation in stable isotope signatures (δ¹³C and δ¹⁵N) of sponges on the Saba Bank

Duyl

Mueller

Meesters³

2018

PeerJ

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“…):

δ^{15} N_{\exp} = \frac{δ^{15} N_{fw} \times f_{x} \times N_{fw} + δ^{15} N_{marine} \times ((), 1 - f_{x}) \times N_{marine}}{N_{fw} + N_{marine}}

where the freshwater δ 15 N composition of riverine (δ 15 N fw ; measured upstream) and tidal marine (δ 15 N marine ) are used to calculate δ 15 N exp based on the N concentration of the respective end‐members ( N fw and N marine ) and the fraction of freshwater present at each sampled point ( f x ). Two δ 15 N marine values (1‰, 5‰) were used to span the reported range from values for “new” N entering the surface ocean (low end‐member) to surface ocean δ 15 N‐TDN values (high end‐member) (Knapp et al , ; Ryabenko et al ; Thibodeau et al ). This δ 15 N range corresponds with those for algae in Moreton Bay at the BRE mouth (0–7‰) (Costanzo et al ) and for Coral Sea TDN and NO 3 − (2–5‰) (Erler et al ; Yoshikawa et al ).…”

Section: Methodsmentioning

confidence: 99%

δ¹⁵N patterns in three subtropical estuaries show switch from nitrogen “reactors” to “pipes” with increasing degradation

Hipsey

Eyre

2018

Limnology & Oceanography

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Ongoing alterations to estuaries by inland agricultural intensification and coastal development could affect their capacity to regulate the flux of excess terrestrial nitrogen (N) to the coastal ocean. Here, a new multiform δ 15 N metric was developed to measure how "pristine," moderately impacted, and highly degraded estuaries recycle (assimilation, mineralization) and remove (denitrification, anaerobic ammonium oxidation) N. Organic (dissolved and particulate, δ 15 N and δ 13 C) and inorganic (nitrate and ammonium, δ 15 N and δ 18 O) N forms were measured over the salinity gradient in the wet and dry season in subtropical estuaries receiving increasing terrestrial N loads (pristine: 16 kg N d −1 , moderate: 150 kg N d −1 , degraded: 630 kg N d −1 ). The difference in the inorganic vs. organic pool δ 15 N composition increased between the pristine (0 AE 2‰), moderate (10 AE 6‰), and degraded (20 AE 8‰) systems, indicating that N recycling decreased as degradation increased. The N 2 O concentrations, NO 3 − dual isotope values, and offsets between "measured" and "mixing expected" δ 15 N values further revealed that microbial processes removed up to 30% of the N load entering the moderately degraded estuary, but only 9% in the highly degraded estuary. Hydrologic differences (depth and flushing times [FTs]) could not fully explain these shifts in N fate between the estuaries and seasons, which instead aligned with nonlinear increases in phytoplankton biomass and light penetration with increasing N loads. These isotopic indicators provide direct evidence that estuaries switch from "reactors" that assimilate and remove terrestrial N to "pipes" that transport N directly to sea as degradation increases.

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“…4b, c) but were negligible overall during the ebb tide (Table 2). Previous studies of reef nutrient fluxes in flumes or other controlled environments have generally shown uptake approaching the limits of mass transfer for NH + 4 (e.g., Atkinson et al, 1994;Cornelisen and Thomas, 2009;Larned and Atkinson, 1997;Thomas and Atkinson, 1997), DIP (reviewed in Cuet et al, 2011b), and, less frequently, for NO x (e.g., Baird et al, 2004); these controlled environments lack some of the confounding processes present in natural reef communities. Yet net release of nutrients (especially NO x ) clearly occurs in situ as concentrations on many reefs exceed those offshore (e.g., Hatcher and Frith, 1985;Leichter et al, 2013;Rasheed et al, 2002), and release rates up to 20 mmol NO x m −2 d −1 , 12 mmol NH + 4 m −2 d −1 , and 2 mmol DIP m −2 d −1 have been measured with in situ studies (Miyajima et al, 2007a, b;Silverman et al, 2012; Wyatt et al, 2012).…”

Section: Rates and Sources Of Benthic Release Of Din And Dipmentioning

confidence: 99%

“…Therefore, the uptake rate has a first-order relationship with nutrient concentration and is a function of water velocity, bottom roughness properties, and diffusion characteristics of the solute (Atkinson, 2011). Due to the dependency of mass-transfer-limited nutrient uptake on flow speed, the local hydrodynamic conditions within a reef directly affect uptake rates of DIN and DIP (Atkinson and Bilger, 1992;Baird et al, 2004;Reidenbach et al, 2006;Thomas and Atkinson, 1997), and these uptake rates can be predicted for a particular reef given sufficient information (Falter et al, 2004;Zhang et al, 2011). However, validating these models with observations from living systems remains a major challenge, as measurements must occur at spatial and temporal scales relevant to reef circulation, and in situ uptake is often confounded by simultaneously occurring biogeochemical processes that release DIN and DIP into the water column (Atkinson and Falter, 2003;Wyatt et al, 2012).…”

Section: Introductionmentioning

confidence: 99%

Tidal and seasonal forcing of dissolved nutrient fluxes in reef communities

2019

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Abstract. Benthic fluxes of dissolved nutrients in reef communities are controlled by oceanographic forcing, including local hydrodynamics and seasonal changes in oceanic nutrient supply. Up to a third of reefs worldwide can be characterized as having circulation that is predominantly tidally forced, yet almost all previous research on reef nutrient fluxes has focused on systems with wave-driven circulation. Fluxes of dissolved nitrogen and phosphorus were measured on a strongly tide-dominated reef platform with a spring tidal range exceeding 8 m. Nutrient fluxes were estimated using a one-dimensional control volume approach, combining flow measurements with modified Eulerian sampling of waters traversing the reef. Measured fluxes were compared to theoretical mass-transfer-limited uptake rates derived from flow speeds. Reef communities released 2.3 mmol m−2 d−1 of nitrate, potentially derived from the remineralization of phytoplankton and dissolved organic nitrogen. Nutrient concentrations and flow speeds varied between the major benthic communities (coral reef and seagrass), resulting in spatial variability in estimated nitrate uptake rates. Rapid changes in flow speed and water depth are key characteristics of tide-dominated reefs, which caused mass-transfer-limited nutrient uptake rates to vary by an order of magnitude on timescales of ∼ minutes–hours. Seasonal nutrient supply was also a strong control on reef mass-transfer-limited uptake rates, and increases in offshore dissolved inorganic nitrogen concentrations during the wet season caused an estimated twofold increase in uptake.

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Heterogeneous dissolved organic nitrogen supply over a coral reef: first evidence from nitrogen stable isotope ratios

Cited by 17 publications

References 36 publications

Spatio–temporal variation in stable isotope signatures (δ¹³C and δ¹⁵N) of sponges on the Saba Bank

Spatio–temporal variation in stable isotope signatures (δ¹³C and δ¹⁵N) of sponges on the Saba Bank

δ¹⁵N patterns in three subtropical estuaries show switch from nitrogen “reactors” to “pipes” with increasing degradation

Tidal and seasonal forcing of dissolved nutrient fluxes in reef communities

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Heterogeneous dissolved organic nitrogen supply over a coral reef: first evidence from nitrogen stable isotope ratios

Cited by 17 publications

References 36 publications

Spatio–temporal variation in stable isotope signatures (δ13C and δ15N) of sponges on the Saba Bank

Spatio–temporal variation in stable isotope signatures (δ13C and δ15N) of sponges on the Saba Bank

δ15N patterns in three subtropical estuaries show switch from nitrogen “reactors” to “pipes” with increasing degradation

Tidal and seasonal forcing of dissolved nutrient fluxes in reef communities

Contact Info

Product

Resources

About

Spatio–temporal variation in stable isotope signatures (δ¹³C and δ¹⁵N) of sponges on the Saba Bank

Spatio–temporal variation in stable isotope signatures (δ¹³C and δ¹⁵N) of sponges on the Saba Bank

δ¹⁵N patterns in three subtropical estuaries show switch from nitrogen “reactors” to “pipes” with increasing degradation