Consistency in coral skeletal amino acid composition offshore of Palau in the western Pacific warm pool indicates no impact of decadal variability in nitricline depth on primary productivity

Abstract

The depth of the thermocline and associated nitricline in the western Pacific warm pool (WPWP) vary over time in response to changes in larger ocean‐atmosphere climate patterns. A shoaling of the nitricline in the WPWP brings nitrate‐rich seawater ( NO3− > 4 μmol kg−1) above the base of the euphotic zone, stimulating primary productivity. Here, we test if decadal variability in the nitricline depth is driving changes in regional primary productivity and source nitrate dynamics. We use the nitrogen isotopic composition (δ15N) of amino acids in the skeleton of a proteinaceous coral collected from the base of the euphotic zone in the WPWP. In proteinaceous corals, as in most organic life, the δ15N of phenylalanine matches that of the ambient nitrate while the δ15N of trophic amino acids reflect subsequent trophic transfer of the nitrogen prior to incorporation into the coral's food, suspended particulate organic matter. Consistency of the trophic position of the coral calculated from the δ15N composition of the coral skeletal amino acids over its 56 yr lifespan suggest that decadal variability in nitricline depth and subsequent shifts in nitrate availability to the euphotic zone have not impacted primary productivity offshore of Palau in the WPWP. This is important when considering the current external forcing of Pacific Ocean climate patterns and the resulting impacts on the global carbon cycle in the Palau region of the WPWP.

Advances in the application of amino acid nitrogen isotopic analysis in ecological and biogeochemical studies

“…Much of the recent work with proteinaceous deep-sea corals has focused on isotope analysis of total skeletal material as a proxy for changes in surface ocean conditions (e.g., Sherwood et al, 2005;Williams et al, 2007;Hill et al, 2014). However, CSIA-AA results can provide unprecedented reconstruction of past ocean conditions (Sherwood et al, 2011(Sherwood et al, , 2014Schiff et al, 2014;Strzepek et al, 2014;McMahon et al, 2015c;Williams et al, 2017). (Loder et al, 2001;Pershing et al, 2001), these authors concluded that changes in nitrate source partitioning may be tied to recent, human-caused changes in global climate.…”

Paired bulk organic and individual amino acid δ15N analyses of bivalve shell periostracum: A paleoceanographic proxy for water source variability and nitrogen cycling processes

“…It is therefore not implausible to assume that environmental conditions would have warranted a change in diet and possibly trophic level in the past. Compound-specific stable isotope analyses of amino acids (CSIA-AA) are increasingly becoming used as a paleoceanographic tool (Sherwood et al, 2011;Batista et al, 2014;Williams et al, 2017;McMahon et al, 2018), in part to address the issue of changes in trophic level influencing changes in  N values of the proxy archive. Nitrogen stored in amino acids is fractionated to different degrees dependent upon the metabolic pathway of the specific amino acid (McClelland and Montoya, 2002;Chikaraishi et al, 2007;Chikaraishi et al, 2009;McMahon and McCarthy, 2016).…”

“…Due to these diagnostic properties, CSIA-AA has been developed in the last several decades for a variety of applications broadly relating to food web dynamics and nitrogen cycling processes (see Ohkouchi et al, 2017 for a review), including using soft tissue of bivalves to reconstruct baseline  15 N conditions (Vokhshoori and McCarthy, 2014) and trophic position (Ek et al, 2018). This technique has also been used in a paleoceanographic context as a proxy for water mass source changes (Sherwood et al, 2011) and to reconstruct the depth of the nitricline through time (Williams et al, 2017). Despite the possible opportunities that  15 N and CSIA-AA  15 N in bivalve shells offers for paleoceanographic studies, there have only been a few reconstructions using these techniques that extend back beyond the instrumental record (Oczkowski et al, 2016;Darrow et al, 2017;Misarti et al, 2017).…”

Population-specific responses in physiological rates of <i>Emiliania huxleyi</i> to a broad CO<sub>2</sub> range

“…Physiological variability makes a population more resilient, increases its ability to persist in variable environments and potentially forms the basis for selection (Gsell et al, 2012;Hattich et al, 2017). It is clear that other environmental factors such as light intensity, temperature, and nutrient concentration affect the responses of physiological rates of individual E. huxleyi strains to changing carbonate chemistry, and thus change the physiological variability within populations (Zhang et al, 2015;Feng et al, 2017). However, different sensitivities and requirements of each strain to the variable environments can allow strains to co-exist within a population in the natural environment (Hutchinson, 1961;Reed et al, 2010;Krueger-Hadfield et al, 2014).…”