Abstract
Global warming and the resulting surface seawater stratification have led to dominance of small phytoplankton, driving substantial changes in planktonic ecosystem structure. This study investigates the composition and nitrogen sources of phytoplankton and other basal resources by analyzing the cosmopolitan copepod
Calanus
spp. in western North Pacific. Carbon isotope ratios of amino acids (δ
13
C
AA
) in
Calanus
spp. tissue revealed geographically distinct basal diets: N
2
-fixing organisms (19.6 ± 5.8%) and heterotrophic bacteria (30.6 ± 5.3%) in the tropical and subtropical Pacific, and dinoflagellates (54.4 ± 10.5%) in the Bering Strait and southern Chukchi Sea. The proportion of dinoflagellates in
Calanus
spp. diets was strongly correlated with the dietary protist contribution estimated using nitrogen isotope ratios of amino acids (δ
15
N
AA
). In addition, nitrogen sources assimilated into total eukaryotic microalgae varied regionally: biologically fixed N
2
in the tropical Pacific (29.2 ± 7.9%), while nitrate was the primary source in the Pacific Arctic Ocean (91.4 ± 2.3%). By linking isotopic compositions to ecological function, this study provides insight into how regional nutrient shifts are altering planktonic food webs, with cascading consequences for marine biodiversity, biogeochemical cycling, and ecosystem resilience in a rapidly changing ocean.