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