The distribution and photobiont characteristics of two lichen species along a resource gradient from the shoreline of Lake Mývatn, Iceland

Abstract

Organisms that move across ecosystem boundaries can create resource gradients in adjacent ecosystems. Midges that emerge from Lake Mývatn, Iceland, are deposited as nutrient-rich carcasses on land, and this generates a nutrient gradient from its shoreline. We contrasted the distribution and photobiont characteristics of two lichen species, Nephromopsis cucullata and Peltigera leucophlebia, in a lava field adjacent to Mývatn along this midge deposition gradient. While the primary photobionts of P. leucophlebia and N. cucullata are green algae, P. leucophlebia also contain nitrogen-fixing cyanobacteria. Because the landscape surrounding Mývatn is generally nitrogen-limited and midge carcasses are nitrogen-rich, we hypothesized that the different photobiont associations would result in contrasting distributions between species and contrasting photobiont characteristics within species along the midge gradient. We found that N. cucullata was at its highest density 100 m from the lakeshore, while P. leucophlebia was more evenly distributed. Both species had a larger photobiont layer thickness near the lakeshore, although N. cucullata had a negative relationship between chlorophyll-a concentration (as measured using a greenness index) and distance, whereas P. leucophlebia showed no pattern. The contrasts between N. cucullata and P. leucophlebia may be related to the presence of cyanobacteria in P. leucophlebia, which provide nitrogen and, in part, decouple P. leucophlebia from the midge nitrogen deposition gradient. This study illustrates that nutrient gradients may alter lichen distributions and photobiont characteristics and suggests that these responses are primarily taxon-specific. Furthermore, it highlights the capacity for the dynamics of aquatic ecosystems to propagate into adjacent landscapes.