## 4 Discussion
### Consequences of FAB Occurrence for Primary Production and Lake Food Web  
Our measured rates of periphyton GPP are within the range that has been described previously for mesotrophic and oligotrophic lakes ([36] and references therein). However, contrary to the findings of a previous study that focused on FAB blooms by _Spirogra_ sp. in a Japanese lake [22], we measured the highest rates of both total and chl _a_-specific GPP on non-FAB rocks in Bear Lake. Several measurements at both the FAB and non-FAB sites showed negative GPP values that were excluded from the analyses (as discussed in Methods). This approach did not affect our results, as such cases occurred at approximately the same frequency at both sites. Higher \(\delta^{13}\)C signatures of periphyton at the non-FAB site support our findings of a higher productivity because they indicate a greater shortage in CO\({}_{2}\), either due to higher photosynthetic rates [57] or a thicker boundary layer [62]. Filamentous green algae such as _Spirogra_ and _Mougeotia_ can exhibit rates of net photosynthesis up to ten times higher than those of _C. glomerata_ (7.7 mg O\({}_{2}\) g\({}^{-1}\) h\({}^{-1}\)) [63], which are highest at 300 umol photons m\({}^{-2}\) s\({}^{-1}\)[64]. The light intensity at our nearshore Bear Lake sampling sites would have been higher than this, potentially leading to an unfavorable energy balance for this species. Low chl \(a\) concentrations and low maximal rates of photosynthesis are physiological acclimations to high-light environments in _C. glomerata_[65]. It is thus possible that light conditions would have been preferable for _C. glomerata_ at greater lake depths, although the cobble substrate on which we observed FAB growth is largely limited in Bear Lake to the shallow nearshore zone [29]. Our chamber measurements excluded the effects of waves on GPP, but this does not explain the lower GPP values of the FABs, as periphyton biomass accumulation is often lower in nearshore zones where wave action suppresses production [66; 67]. Although seasonal analyses of GPP are limited due to small sample sizes, the measured GPP rates did not differ significantly between months at either the FAB or non-FAB site. Whilethe mean GPP rates at the FAB site declined steadily from June (217 \(\pm\) 44 mg C m\({}^{-2}\) h\({}^{-1}\)) to July (145 \(\pm\) 58 mg C m\({}^{-2}\) h\({}^{-1}\)) to October (75 \(\pm\) 52 mg C m\({}^{-2}\) h\({}^{-1}\)), they remained substantially lower than the measured rates at the non-FAB site (overall mean = 451 \(\pm\) 129 mg C m\({}^{-2}\) h\({}^{-1}\)). It is therefore possible that earlier springtime GPP rates were higher at the FAB site than those that were measured in our three campaigns, but there is no evidence to indicate that they could have been high enough to influence the overall differences in the GPP rates that were observed between the FAB and non-FAB sites. Assuming constant non-FAB GPP rates, the springtime FAB GPP would need to surpass 1400 mg C m\({}^{-2}\) h\({}^{-1}\), which is an order of magnitude higher than the mean June-measured GPP rates, as well as far above the range of rates that are typically reported in the literature ([67] and references therein). The implications of lower GPP by FABs for the food web of Bear Lake are yet unclear. Littoral-benthic resources are an important support for aquatic food webs [67; 68; 21], including those in Bear Lake, where phytoplankton production is minimal [69]. Elevated CR rates in the non-FAB treatment compared to the FAB treatment (Figure 4C) indicate that a large fraction of the carbon fixation in these assemblages likely fuels microbial production and respiration, although it cannot be taken for granted that such carbon is effectively transferred up the food web to higher trophic levels [70], as variable bacterial growth efficiencies can also result in a loss of carbon as respiration to the water column ([32] and references therein, [71]).