LAUSR

Abstract 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 δ 13 C than copepods at sites near the inlets, and that variations in the δ 13 C values of copepods mirror that of the SPOM. Copepod δ 13 C values ranged from −27.8‰ to −19.4‰, and that of SPOM from −26.8 to −20.3‰. Mean copepod δ 13 C value (−21.8‰ ± 0.4) at a site close to the ocean was the highest, whereas the lowest value (−24.4‰ ± 0.5) occurred at a site located at the mouth of St. Martin River. δ 15 N copepod values varied from 2.9‰ to 13.0‰ (mean = 9.2‰ ± 0.1); that of SPOM was −0.58 to 10.51‰ (mean=5.7‰ ± 0.2). Reduction in the δ 15 N mean values from a site near the Ocean City inlet (10.2±0.4‰) to a site close to the tributaries (8.5±0.6‰) suggests that copepods at the site near Ocean City inlet likely fed on food items that contained more marine phytoplankton with enriched δ 15 N and heterotrophic protists, than copepods at a site near the river mouth. The highest (−21.9‰ ± 0.4) and lowest (−25.8‰ ± 0.3) mean copepod δ 13 C values were observed in summer 2015 and winter 2017, whereas the highest (10.8‰ ± 0.2) and lowest (7.1‰ ± 0.7) mean copepod δ 15 N values were recorded in winter and fall 2016, respectively. δ 15 N enrichment between copepods and SPOM ranged from 1.0‰ to 8.9‰ (mean: 3.4‰). The highest enrichment observed in February 2016 stemmed from a reduction in SPOM δ 15 N values, after a major storm event, which was unaccompanied by a decrease in Acartia δ 15 N. Copepod and SPOM δ 13 C and δ 15 N 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.