What Shapes Life in the Rock Pools of Isle Royale? Waves, Runoff and Bedrock

Along the northeastern shore of Isle Royale in Lake Superior, more than 70,000 small rock pools look like tidepools but hold amphibians and insects instead of starfish. A National Park Service article summarizes a study of them by an NPS aquatic ecologist from the Great Lakes Network and university partners that overturned several expectations.

Image from the National Park Service's page: Lake Superior seen from Isle Royale National Park.

The pools

More than 70,000 coastal rock pools are scattered along 30 miles of the park's northeastern shoreline. Each is a small, shallow patch of habitat with its own physical traits, water chemistry and species, and together they form a mosaic of biologically rich ecosystems. From April to October 2010 the team visited a set of pools about once a month to see how they differ and to study the midge family Chironomidae, the largest family of aquatic insects and food for fish, birds and other species. Midges spend their egg, larval and pupal stages in the water and then emerge as flying adults, shedding skins on the surface that can be sampled and identified.

The team studied 16 pools at four sites on Isle Royale and two small nearby islands: at each site, two "splash" pools near the lake that receive wave wash, and two "lichen" pools farther upslope that receive only rain and a little runoff. Each pool was visited six times for measurements and water and skin samples.

What they found

The metals surprised the team. With no mining or industry nearby, they concluded the metals come from the island's volcanic bedrock, which is rich in some metals (Indigenous peoples mined copper on the island in prehistoric times) and releases them as it weathers; runoff carries them downslope. In some lichen pools aluminum and copper reached levels high enough to harm sensitive aquatic invertebrates, which suggests bedrock weathering can change the midge community.

Image from the National Park Service's page: skins left on the water's surface as midges emerged, photographed by an NPS ecologist.

The samples held at least 102 species. Of the 29 abundant enough to analyze, eight were found only in lichen pools, 12 only in splash pools and nine in both. The team expected different species in the two zones but not some of the ones found. A colleague noticed caddisfly larvae of a species usually found in flowing water in a splash pool next to the lake, and some midge skins were also typical of flowing water. That makes sense in hindsight: waves constantly wash the splash pools with cold, low-nutrient water, which mimics a fast headwater stream.

Why it matters

Both findings, wave action in splash pools and bedrock weathering in some lichen pools, challenge how NPS researchers and managers think species respond to their surroundings. Trace metals are not always included in water chemistry analyses, so the weathering effect could easily have been missed. The ecologist said species do not always respond as people expect, so stepping back to reconsider a system matters, and called the pools a healthy mosaic with probably a lot of capacity for resilience.

Image from the National Park Service's page: a freighter near Passage Island.

Though remote, the park is vulnerable to human impacts. Its ecosystems are affected by climate change, and a nearby international shipping lane makes oil spills a risk; the park's highest densities of rock pools and sensitive species lie beside that lane. Results like these will help managers plan for climate change and for any marine incident.

Sources

• National Park Service, Great Lakes Network, What Drives Diversity in Rock Pools at Isle Royale National Park?, summarizing Egan, A., Burge, D.R.L., Edlund, M.B., Lafrancois, T. and Ferrington, L.C., Jr., "Distribution of water chemistry, trace metals, and Chironomidae in coastal freshwater rock pools," Freshwater Science 43(2), 2024 (the first author is an aquatic ecologist with the National Park Service's Great Lakes Network); the photographs of people left out; rewritten in hubnx's own words.