Temporal and spatial variability of <sup>13</sup>C/<sup>12</sup>C and <sup>15</sup>N/<sup>14</sup>N in pelagic biota of Prince William Sound, Alaska

Abstract

Stable isotope ratios of carbon and nitrogen were used to identify seasonal and spatial patterns in carbon and nitrogen and to determine source of energy (Prince William Sound (PWS) versus the Gulf of Alaska (GOA)) for juvenile fishes in PWS. PWS-wide samples of bulk net zooplankton (all noncalcareous zooplankton collected in 335-µm-mesh nets), individual late copepodid stage of the large herbivore Neocalanus cristatus, juvenile Pacific herring (Clupea pallasi), and juvenile walleye pollock (Theragra chalcogramma) were collected in spring, summer, and fall in 1994 and 1995. For bulk zooplankton and N. cristatus, there was a strong 13C/12C gradient but weak 15N/14N gradient within PWS and GOA. Zooplankton 15N/14N was positively correlated with 13C/12C during the phytoplankton bloom but was not correlated during the zooplankton bloom, suggesting a decoupling of nitrogen and carbon cycles. Plankton isotopic signatures suggested a diagnostic 13C/12C for GOA carbon. For juvenile fishes and diapausing copepods in PWS, 13C/12C varied between years, suggesting that the origin of carbon differed between years (GOA more so in 1995 than in 1994). Use of a natural stable isotope tracer provided evidence for biophysical coupling via inferred fluctuations in oceanographic processes.