## 4 Discussion
### Potential Causes of FAB Occurrence  
An apparent recent rise in FABs has been linked to a variety of drivers in lakes around the world, including climate change, eutrophication (including groundwater pollution), and food web effects [11]. We considered that climate may have played some role in the occurrence of FABs in Bear Lake, as periphyton production can respond positively to warming [14; 45]. _C. glomerata_, the dominant species in Bear Lake FABs, has its photosynthesis temperature optimum at 28-31 \({}^{\circ}\)C [46] and modelling predicted an earlier spring growth with surface water warming [47]. Furthermore, ice cover period may be an important measure for considering recent FAB formation developments, as littoral ice may physically scrape periphyton (and associated nutrient matrices) from nearshore rocks, resetting periphyton colonization on rocks and thus physically reducing FAB formation. Coupled with the recent increase in summertime temperatures, there is a plausible mechanism whereby the climate warming of Bear Lake assists the development of FABs. _C. glomerata_ often dominates the benthic flora of eutrophic waters and it has become a nuisance on lake shores worldwide as a result of anthropogenic eutrophication ([48] and references therein). Elevated concentrations of soluble phosphate that were associated with cultural eutrophication were considered to be responsible for the _Cladophora_ bloom occurrences in the lower North American Great Lakes in the 1950s to 1980s [44], and are still responsible for their current nuisance development [49]. While not yet considered a nuisance in Bear Lake, its presence there may potentially indicate local eutrophication [13]. All the \(\delta^{15}\)N values that were measured were higher than 0 % and thus may indicate an influence of livestock or human waste on nearshore littoral areas [50; 51; 52], although within-group variability of \(\delta^{15}\)N can be high [52; 53]. While livestock numbers in the area are in decline, and there has been no upward trend in local flood irrigation that could wash terrestrial nutrients to the lake's littoral zone, tourism and associated construction has risen in recent decades (pers. obs.). Eutrophication via lacustrine groundwater discharge [54] may also provide nutrients directly to the sediment-water interface, thus promoting FABs [12], and it is possible that a highly variable but generally declining trend in Bear Lake's water levels [29] may be have exposed local groundwater discharge to areas that are closer to shore, making the occurrence of FABs more readily visible. Such locally and temporary increased nutrient availability, however, is difficult to directly measure from monitoring the nutrient concentrations in the water. Tissue nutrient concentrations of benthic primary producers such as macrophytes or filamentous algae that integrate over longer periods are therefore a useful measure for detecting such hidden nutrient inputs. In June 2021, the tissue N contents of filamentous algae from the FAB site were significantly higher than those from the non-FAB site (Figure 5), which could also indicate a higher N availability at the FAB site, although it may also be an effect of a higher proportion of inorganic material in the dry weight of the non-FABs. We also considered that top-down controls can play a role in FAB occurrence in lakes [11]. Specifically, the disappearance of periphyton grazers such as snails may release periphyton from biotic controls on their biomass [55]. Alternatively, the selective grazingof epiphytotic diatoms growing on _Cladophora_ mats could potentially promote _Cladophora_ growth [16; 56]. Indeed, the effects of herbivore grazing of periphyton have previously been associated with increased periphyton biomass-specific productivity through fecal fertilization and increased light supply to periphyton [16; 57].