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.