Influence of the 2013-2015 marine heatwave on zooplankton community structure in the lower Cook Inlet, Alaska

In the past 30 years, Cook Inlet, Alaska (CI) has experienced significant changes in environmental conditions from anthropogenic and atmospheric events including the Exxon Valdez Oil spill, multiple ENSO cycles, and the recent 2014-2016 Pacific marine heatwave known as “the Blob”. The latter event coincided with the appearance of several well-documented marine species (e.g. ocean sunfish, skipjack tuna, and several species of copepods) common along the coasts of California to Washington in the Gulf of Alaska, and CI. As part of the Gulf Watch Alaska long-term monitoring program, CI zooplankton were collected to understand how these disturbances affect ecological patterns in zooplankton associated with the nearshore marine system. From 2012 to 2017, zooplankton were collected monthly along five nearshore transects across CI with a 0.6 m bongo net (333 µm mesh) towed vertically from 50m to the surface (n = 396). Tows were subsampled and microscopically enumerated for species composition and abundance. GAMs showed that abundance peaked mid-June (2533.70±189.64 no. m^-3) and gradually declined to a low in January (9.40±224.80 no. m^-3). Of the 237 species identified, 77 of the most abundant species (>5% presence) were used in an indicator species analysis (ISA) to show seasonal shifts in community composition. In early spring, the community was comprised of Neocalanus spp. copepods and ichthyoplankton (e.g. Clupea pallasi). Progressing into late spring, the community switched to a larvae (barnacles, polychaetes, euphausiids) dominated structure that also included copepods common to the region (Pseudocalanus spp. and Calanus marshallae). In the late summer through early fall, communities were comprised of gelatinous zooplankton. A year after the marine heatwave dissipated (2017), the community dominated by large copepods and ichthyoplankton was replaced by a community comprised of the larvae and smaller common copepods. Beginning in 2016, the last year of heatwave, a fall community characterized warm-water associated copepods (Paracalanus parvus and Corycaeus anglicus) increased significantly (9 events 2012-2015; 20 events in 2016) and continued to dominate the communities in the fall of 2017 (12 events), suggesting effects of anomalously warm-waters may persist after the marine heatwave dissipated.