*4.2. Algae*

*Dictyosphaeria cavernosa* was once the dominant algae species in Kane'ohe Bay, responsible for one ¯ of the first well-studied reef phase shifts from coral-dominated to algae dominated [20]. Following the phase-shift reversal, the algae persisted in the bay due to overfishing of herbivorous fish that would have placed grazing pressure on the species [20]. *Dictyosphaeria cavernosa* remained abundant in Kane'ohe Bay, averaging 16% total cover during a 1996–1997 survey [ ¯ 20]. The findings of the 2000 survey indicate the percent cover of *Dictyosphaeria* spp. remained at a comparable level three years later at the fringing reef (16.7 ± 1.5%). In 2006, following an unusually rainy period, decreased irradiance combined with slow spring growth rates for the species caused *D. cavernosa* to effectively disappear from Kane'ohe Bay [ ¯ 34]. Immediately following the rapid decline, reefs nearby Moku o Lo'e averaged 0–4% total cover of *D. cavernosa* [35]. In 2018, twelve years later, the prevalence of *D. cavernosa* has remained greatly diminished at this fringing reef (1.1 ± 0.3%), suggesting an enduring phase shift reversal.

The invasive species *G. salicornia* was introduced to Kane'ohe Bay in the 1970 ¯ s and quickly spread, overgrowing and smothering reef-building corals [36]. The invasive algae has since decreased over the past few years as a result of biocontrol [37], manual removal [38], and increased grazing from *Chelonia mydas,* the green sea turtle [39]. The managemen<sup>t</sup> efforts and return of *C. mydas* to Kane'ohe Bay likely ¯ explain why the once dominant macroalgae was not observed during the 2018 survey.

Like *G. salicornia*, *Kappaphycus alvarezii* (formerly *Eucheuma striatum*) was introduced to Kane'ohe ¯ Bay in the 1970s [40] and had spread across the southern and central bay by 1996 in a near-cosmopolitan distribution [41]. A total percent cover of 0.33 ± 0.3% in the 2000 survey was slightly higher than the mean 0.06 ± 0.02% cover found at four shallow fringing reefs in the central bay in 1996 [41]. Amidst fears of further spreading, preliminary managemen<sup>t</sup> options for *Kappaphycus* spp. were assessed in 2002 [42]. Divers used an underwater vacuum cleaner and outplanted juvenile urchins (*Tripneustes gratilla*) to remove and control the species in 2011–2013, leading to an 85% decrease in invasive macroalgae across sites [38]. Management efforts have continued to be successful as *K. alvarezii* was not observed at the study site during the 2018 survey.

Despite *Dictyosphaeria* spp., *G. salicornia,* and *K. alvarezii* all decreasing or disappearing from the reef, a total increase in algal cover from 2000 to 2018 was observed, mainly due to the increase in 'non-coral substrate'. It should be noted that 18.6 ± 0.8% of the non-coral substrate from the 2018 survey was crustose coralline algae (CCA). CCA was not categorized or differentiated from 'encrusted corals' in the 2000 study. Thus, the percent cover of total algae as well as non-coral substrate is inflated in the 2018 data and likely the 2000 data as well. Unlike turf and macroalgae, CCA promotes coral recruitment and recovery [43] and would have ideally been separated into its own category.

The high percentage of non-coral substrate in 2018 (55.6 ± 0.9%) was also impacted by the prevalence of (perhaps short-lived) turf on the tips of *P. compressa* and *M. capitata.* The tips of these reef-building corals were susceptible to warming events and air exposure at extreme low tides as the 2018 survey was conducted in late July following a warm period and spring tides (Figure 6). Observed spatial differences within benthic communities showed certain sections of the reef were more susceptible to algal growth. During the 2018 survey, the northern portion of the reef exhibited higher levels of non-coral substrate than the southern portion of the reef (Figure 5). In addition to spatial variations in low tide air exposure, differences in temperatures could explain this occurrence as corals near the northern end experienced increased thermal stress (2018 summer midday average (11:00–16:00) temperature 27.72 ± 0.94) compared to corals at the southern end (2018 summer midday average temperature 27.48 ± 0.96). This difference highlights the importance local microclimates have on coral communities.

**Figure 6.** Reef Air Exposure. (**a**) Reef exposed during low tide in Kane'ohe Bay (Picture Credit: KDB). ¯ (**b**) Tips of a pale *P. compressa* colony covered with turf (Picture Credit: KAB).
