Inherent tolerance of extreme seasonal variability in light and salinity in an Arctic endemic kelp (Laminaria solidungula).

The kelp Laminaria solidungula is an important foundation species in the circumpolar Arctic. One of the largest populations of L. solidungula in the Beaufort Sea occurs in Stefansson Sound, off the north coast of Alaska. We surveyed kelp populations in the Stefansson Sound Boulder Patch and found that inshore sites in close proximity (3.5 km) to river input and increased turbidity exhibited lower sporophyte densities (0.36 ± 0.44 · m-2 ) than more offshore sites (>7 km) to the west (0.72 ± 0.48 · m-2 ) and east (4.72 ± 1.51 · m-2 ). We performed culture experiments to examine the possible combined effects of salinity and light on microscopic sporophyte production. Gametophytes cultured in the low salinity treatment (10) were unable to produce sporophytes regardless of light level. The highest light level tested (40 µmol photons · m-2  · s-1 ) produced the greatest sporophyte densities (0.037 ± 0.08 · mm-2 ) at a salinity of 30. Subsequent experimental work on the effect of salinity on microscopic stages revealed that haploid stages were not capable of producing sporophytes at a salinity of 10, but 3-month-old microscopic sporophytes were able to persist in the lower (10 and 20) salinity treatments. Although L. solidungula sporophytes have apparently acclimated to extreme salinity (<5-33) and light variations, the vulnerability of haploid microscopic stages to reduced salinity has the potential to affect future populations as the timing and magnitude of freshwater input to the Arctic Ocean changes.

Physiological Responses of an Arctic Crustose Coralline Alga (Leptophytum foecundum) to Variations in Salinity.

In the Beaufort Sea, Arctic crustose coralline algae (CCA) persist in an environment of high seasonal variability defined by naturally low pH ocean water and high magnitude freshwater pulses in the spring. The effects of salinity on the CCA Leptophytum foecundum were observed through a series of laboratory and field experiments in Stefansson Sound, Alaska. We found that salinity (treatments of 10, 20, and 30), independent of pH, affected L. foecundum physiology based on measurements of three parameters: photosynthetic yield, pigmentation, and calcium carbonate dissolution. Our experimental results revealed that L. foecundum individuals in the 10-salinity treatment exhibited an obvious stress response while those in the 20- and 30-salinity treatments were not significantly different for three parameters. Reciprocal in situ transplants and recruitment patterns between areas dominated by CCA and areas where CCA were absent illustrated that inshore locations receiving large pulses of freshwater were not suitable for CCA persistence. Ultimately, spatially and temporally varying salinity regimes levels affected distribution of CCA in the nearshore Arctic. These results have implications for epilithic benthic community structure in subtidal areas near freshwater sources and highlight the importance of salinity in CCA physiology.

Recruitment tolerance to increased temperature present across multiple kelp clades.

Kelp systems dominate nearshore marine environments in upwelling zones characterized by cold temperatures and high nutrients. Worldwide, kelp population persistence and recruitment success generally decreases with rising water temperatures coupled with low nutrients, making kelp populations vulnerable to impending warming of the oceans. This response to climate change at a global scale, however, may vary due to regional differences in temperature variability, acclimation, and differential responses of kelp species to changing conditions. Culture experiments were conducted on 12 eastern Pacific kelp taxa across geographic regions (British Columbia, central California, and southern California) under three nitrate levels (1, 5, and 10 µmol/L) and two temperatures (12°C and 18°C) to determine sporophyte production (i.e., recruitment success). For all taxa from all locations, sporophytes were always present in the 12°C treatment and when recruitment failure was observed, it always occurred at 18°C, regardless of nitrate level, indicating that temperature is the driving factor limiting recruitment, not nitrate. Rising ocean temperatures will undoubtedly cause recruitment failure for many kelp species; however, the ability of species to acclimatize or adapt to increased temperatures at the warmer edge of their species range may promote a resiliency of kelp systems to climate change at a global scale.

Effects of Zoospore Aggregation and Substrate Rugosity on Kelp Recruitment Success.

Successful kelp recruitment is important for kelp population persistence and associated kelp forest communities. The proximity of settled kelp zoospores is a known requirement for successful kelp recruitment and proximity can be increased as zoospores aggregate. Substrate rugosity can also be an important factor affecting macroalgal settlement and recruitment in wave-swept areas, and may affect kelp recruitment by aggregating zoospores. In this study, kelp zoospores were cultured at different levels of small-scale aggregation and kelp recruitment was quantified. Sporophyte production significantly increased as zoospores became more aggregated indicating that processes that aggregate kelp zoospores have the potential to enhance kelp recruitment. A 13-month field experiment demonstrated differential kelp recruitment onto settlement plates that mimicked surface rugosities of two common rock types within Stillwater Cove, Carmel Bay in central California (Carmelo Formation sandstone and Santa Lucia granodiorite). Significantly more kelp recruited to molds mimicking granodiorite over the yearlong study (granodiorite = 2.7 recruits ± SE 0.50, sandstone = 1.2 recruits ± SE 0.51). There was a significant difference in recruitment between seasons and this variability was due to the fact that spring had the highest average number of kelp recruits per mold. However, the interaction between substrate and season was not significant. This study emphasizes the importance of kelp zoospore aggregation on kelp recruitment and demonstrates that small-scale rugosity affects kelp recruitment.