Carbon and nitrogen stable isotopes of copepods in a tidal estuarine system in Maryland, USA

The carbon and nitrogen stable isotope ratios of copepods, particularly Acartia tonsa and suspended particulate organic matter (SPOM) samples, collected seasonally (2014 to 2017) from 13 sites in the tidal estuaries of Maryland were analyzed. We hypothesized that copepods at sites close to the mouths of lagoon tributaries have more depleted delta C-13 than copepods at sites near the inlets, and that variations in the delta C-13 values of copepods mirror that of the SPOM. Copepod delta C-13 values ranged from -27.8 parts per thousand to -19.4 parts per thousand, and that of SPOM from -26.8 to -20.3 parts per thousand. Mean copepod delta C-13 value (-21.8 parts per thousand +/- 0.4) at a site close to the ocean was the highest, whereas the lowest value (-24.4 parts per thousand +/- 0.5) occurred at a site located at the mouth of St. Martin River. delta N-15 copepod values varied from 2.9 parts per thousand to 13.0 parts per thousand (mean=9.2 parts per thousand +/- 0.1); that of SPOM was -0.58 to 10.51 parts per thousand (mean=5.7 parts per thousand +/- 0.2). Reduction in the delta N-15 mean values from a site near the Ocean City inlet (10.2 +/- 0.4 parts per thousand) to a site close to the tributaries (8.5 +/- 0.6 parts per thousand) suggests that copepods at the site near Ocean City inlet likely fed on food items that contained more marine phytoplankton with enriched delta N-15 and heterotrophic protists, than copepods at a site near the river mouth. The highest (-21.9 parts per thousand +/- 0.4) and lowest (-25.8 parts per thousand +/- 0.3) mean copepod delta C-13 values were observed in summer 2015 and winter 2017, whereas the highest (10.8 parts per thousand +/- 0.2) and lowest (7.1 parts per thousand +/- 0.7) mean copepod delta N-15 values were recorded in winter and fall 2016, respectively. delta N-15 enrichment between copepods and SPOM ranged from 1.0 parts per thousand to 8.9 parts per thousand (mean: 3.4 parts per thousand). The highest enrichment observed in February 2016 stemmed from a reduction in SPOM delta N-15 values, after a major storm event, which was unaccompanied by a decrease in Acartia delta N-15. Copepod and SPOM delta C-13 and delta N-15 values, displayed similar spatial patterns and were positively correlated; enrichment at sites near the ocean and depletion near the river mouth indicating that SPOM composition influenced copepods diets in the lagoons. (c) 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

The study analyzed the carbon and nitrogen stable isotope ratios of copepods and SPOM samples collected from different sites in the tidal estuaries of Maryland, revealing that the isotope values of copepods are influenced by their location and food sources.