Kapoʻo Tidepool · Oʻahu

What happens to the water after you rinse off?

In collaboration with Mālama Pūpūkea-Waimea, a UH Mānoa research team is studying how the public beach shower affects water quality in the tidepool just downhill — and whether native plants can help.

A small shower, a sensitive ecosystem

Kapoʻo is a semi-enclosed tidepool within a designated Marine Life Conservation District (MLCD). Because of its shape and limited connection to the open ocean, water that enters the pool stays for a long time — especially in summer. That makes it uniquely sensitive to what flows in from the surrounding land.

Every time someone uses the beach shower, freshwater carries soap residue, sunscreen, and nutrients across the sand toward the tidepool. We don’t yet know how much this matters. That’s what we’re here to find out.

GRYVOZE digital flow meter installed on shower spigot — Digital flow meter installed on one of four shower spigots, logging cumulative water volume discharged toward the tidepool.

Runoff corridor between shower and ocean at low tide — The runoff corridor between the shower and water’s edge at low tide. This is the path water takes toward the ocean.

The Sampling Design

Six points. Shower to ocean.

Water and sand samples are collected at six locations along the path from the shower outlet to the open ocean reference site, at both high and low tide. Tracking how water quality changes across this gradient tells us how far the shower’s influence reaches — and how much the tidepool’s chemistry differs from the open ocean.

Shower outlet

Runoff path

Tidepool edge

Tidepool center

Barrier wall

Open ocean

← High input Reference →

Methods

What we’re measuring

Benthic Surveys

Photo surveys document what’s living at each gradient point — coral, coralline algae, fleshy macroalgae — linking water quality to ecosystem health before and after the native plant installation.

Water Chemistry

Nutrients, dissolved organic matter, and pollutants are measured at each gradient point. Elevated inputs from shower use can fuel algae growth and shift the reef away from coral — a sign of a stressed ecosystem.

Flow Rate

Digital meters on all four shower spigots log how many gallons are draining into the surrounding area. Volume of water used × concentration of nutrients in water = estimated load on the ecosystem.

The Natural Experiment

Before and after native plant installation

Native Ae ae (Bacopa monnieri) — a coastal wetland plant native to Hawaiʻi — will be planted along the shower runoff corridor once baseline data collection is complete. Ae ae is known to filter stormwater and may take up certain contaminants into its root structure before they reach the sea. By documenting conditions before and after planting, we can measure whether the intervention actually changes what reaches the ocean.

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