2. Methods

Ocean Niño Index climate events are defined here as starting in the year of the end of the first La Niña closely paired with an El Niño by ≤2 months separation, where anomalies for both El Niño and La Niña exceed 1.0 on the ONI scale for 5 months or longer (available from tinyurl.com/ENSONOAA). By that definition, the starting points of climate regime shifts from the literature get changed to earlier years in some cases; 1974 rather than 1977 and 1999 rather than 2001. Since the 1989 regime shift involved only the pairing of one La Niña after a strong El Niño, it remains starting at 1989. The regime shift of 2011 [2] is designated for the end of the first of two consecutive La Niñas paired with a strong El Niño.

3. Results

An asterisk indicates trace abundance.

occurred throughout all years.

same was true for Brachiopoda.

Most of the 1077 taxa were present during all climate regimes in the Strait of Georgia, documented in 3865 dives. When aligned with climate regime event-years, 171 selected rare and abundant species showed correspondence to the Ocean Niño events (Table 1). Only rare taxa were undetected during entire regimes. For the most abundant taxa, patterns of increased or decreased abundance correspond to the years defining climate regimes, suggesting the possibility that causal relations may one day be determined. Abundance data for the 171 selected species are in Table 1 for the entire Strait of Georgia region, including Howe Sound.

Seabed Biodiversity Shifts Identify Climate Regimes: The 2011 Climate Regime Shift and Associated Cascades

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Among the Orchophyta the Desmarestia (acidic) species occur irregularly but are of note in recent years since 1999. Neogarum jumped in abundance during the 1999 regime, whereas a search anomaly with regard to study of widespread urchin barrens and kelp recovery resulted in anomalously high abundance estimates for this kelp during 2014–2017. Limiting a data compilation to first dives at each site yielded different results, with Neoagarum absent (urchin barrens) at over half of all sites for 2014–2017. Among the Rhodophyta there were seven genera (Porphyra, Hildenbrandia, Clathromorphum, Callophyllis, Mazzella, Constantina and Opuntiella) that peaked during the 1999

Among the Porifera, Leucosolenia and Adocia were mainly abundant during the 1999 regime, whereas Pachychalina and Myxilla were abundant in both the 1989 and 1999 regimes. Plocamia was abundant mainly in the 1974 and 1989 regimes, in contrast to Cliona, for example, which

In the Cnidaria, Cribrinopsis was highest in abundance during the 1999 regime; few have been seen in recent years. Peachia was also most abundant during the 1999 regime. Similarly, Pachycerianthus was abundant during the 1999 regime, declining during the 2011 regime; Ptilosarcus was also most abundant during the 1999 regime. Halipterus was absent during the 1989 regime, abundant during the 1999 regime, then dropped out again in 2014. Stylantheca was also steady in abundance until 2014. The jellies Cyanea, Aurelia, Aequorea and various hydromedusae were especially abundant during the 1999 regime, as was the case for ctenophores.

Rare species of nemerteans were absent in the 2011 regime, as with sipunculid worms and some annelid worms. An exception is Protula pacifica, which was least abundant during the 1989 regime. Bryozoans were either lower in abundance or absent in the 2011 regime. The

In the Mollusca many species were reduced in abundance (some absent) in the 2011 regime. An exception is the very obvious species Pododesmus machroschisma, which was higher in abundance during the 1999 regime, but still remained abundant in the 2011 regime, typical for many common species not included in this table for which abundance does not fluctuate in any pattern discernable with regime shifts. Distinction of Mopalia spp. among ten different species was not achieved until 1996, yet the abundance of these species dropped in the 1999 and 2011 regimes. The common and obvious species Ceratostoma foliatum is typical of these

regime. Note that seaweed dive identification had not advanced prior to the 1989 regime.

Using search programs for a long-term SCUBA taxonomic database (3865 dives) for Strait of Georgia seabed sites [22], 1077 taxa were screened to select 171 rare or highly abundant taxa and to present the data according to climate regime periods as defined above. The majority of taxa was more uniformly abundant through the survey period and obscured any trends visible from scanning just the 171 species. We present taxon data in tabular form so that relations of biodiversity data to Ocean Niño event-based regime shifts can be visualized.

We used these biodiversity surveys to compare the abundance of sunflower stars and green urchins in Howe Sound through time, the same survey methods used for the long-term database. Surveys were conducted on SCUBA using the roving diver technique at depths from 7 to 30 m between 1984 and 2016. The relative abundance of each species observed during a dive was estimated visually and grouped into a numerical category: none = 0; few ≤10; some ≤25; many ≤50; very many ≤100; abundant ≤1000; very abundant = thousands. To calculate annual averages, maximum values for each category were used (3000 for "very abundant"). Subsequent to SSWD and the green urchin explosion, observations of Neoagarum fimbriatum abundance and spot prawn nursery settlement have enabled interpretation of cascade effects that relate to climate regimes.

Geographic locations of dives within the Strait of Georgia (Figure 1) shifted through the years and research priorities may have influenced the abundance averages for some years. Many of the species, however, were not the focus of special dive searches and were listed in dive summary taxon records as a matter of routine, so that most abundance records can be taken as derived by standard methods. In recent years, focus on location and abundance of Neoagarum versus green sea urchins Strongylocentrotus droebachiensis in Howe Sound has required careful interpretation.

Spot prawn abundance was quantified by monitoring spot prawn nursery settlement [20]. Using settlement records, each site was scored as urchin barren or not, based on whether Neoagarum was present. At sites lacking records of urchins or Neoagarum, juvenile prawn counts greater than zero were assumed not to be an urchin barren. However, zero counts for prawns did not indicate an urchin barren, as zero counts frequently occur in dense Neoagarum [20].
