New study explores survival strategies by invasive macroalgae and native limu in spring-fed nearshore ecosystems

In this project, University of Hawaiʻi at Mānoa botanists measured survival of invasive macroalgae and limu (Hawaiian macroalgae) in brackish nearshore springs (submarine groundwater discharge, SGD), where the tidal cycle drives extreme daily fluctuations in salinity.
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Our recent work Integrated physiological response by four species of Rhodophyta to submarine groundwater discharge reveals complex patterns among closely-related species, was recently published in Scientific Reports. In this project, University of Hawaiʻi at Mānoa botanists measured survival of invasive macroalgae and limu (Hawaiian macroalgae) in brackish nearshore springs (submarine groundwater discharge, SGD), where the tidal cycle drives extreme daily fluctuations in salinity. During low tide, these plants and the nearshore reefs they live on are inundated with fresh water from our subterranean aquifer. Fresh-water springs provide valuable nutrients to these plants and nearshore ecosystems. High tides, however, bring salty, nutrient-poor ocean water. The flow of fresh water is suppressed by the high tide. How do these plants survive such extremes? Are all limu the same?

Initial characterization showed that invasive species “gorilla ogo” (Gracilaria salicornia) and the “spiny seaweed” (Acanthophora spicifera) are particularly abundant in nearshore spring conditions found at Waiʻalae ʻIki, near Wailupe Beach Park on Oʻahu’s south shore. Yet, native limu maneʻoneʻo (Laurencia dendroidea) and a presumed introduction and close relative of limu manauea, Gracilaria perplexa, were sparse.  Why are these natives less common in the nearshore spring region of Waiʻalae ʻIki, a native habitat?

To answer these questions, we employed tools of plant physiology that allows us to assess how a species exists in these extreme tidal conditions. Our team of researchers measured cellular responses in multiple ways including a novel tool: measures of tissue water potential regulation. In tissue water potential regulation, plants change the concentration of solutes within their cells in response to changes in external conditions. We measured if plants changed in response to the stress of changing external salinities. Much like a raisin that swells when immersed in water, these plants, too, are affected by changes in the surrounding seawater. Plants with limited ability to respond to salinity changes burst cells or lose water completely, leading to tissue loss or death. 

These results demonstrate that macroalgae are regulating solute concentrations within their cells in response to external changing salinities. Even more intriguing are the different strategies measured for two invasive species: Gorilla ogo photosynthesized most rapidly under the “stressful conditions” found nearest to the springs, likely allowing this species to take advantage of spring borne nutrient sources. The spiny seaweed, on the other hand, had high rates of photosynthesis on all regions of the reef. Both species had thinner cell walls under spring conditions, likely an adaptation that allows their cells to resist bursting. Further, gorilla ogo also had smaller cells in spring conditions, another likely adaptation. These morphological and physiological traits likely contribute to the success of the invasive algae of Hawaiʻi’s spring-fed nearshore reef regions. 

Why limu maneʻoneʻo and G. perplexa were not found in the spring-influenced region left us with further questions. We do know that these species had similar solute concentrations in their cells offshore, and limu maneʻoneʻo had photosynthetic rates nearly as high as the spiny seaweed. G. perplexa, on the other hand, was the slowest photosynthesizer of the four species studied. Are these species simply unable to compete with the invasive species, are fish eating them before they can be measured, or are they unable to tolerate extreme changes in salinity? As always, more work remains to be done. 

Because humans continue to alter watersheds and SGD, these spring-fed ecosystems are often impacted. Changes in water quality and quantity are likely already influencing limu and macroalgae on our nearshore reefs, and having cascading effects on nearshore food webs and coral cover. It is critical that we understand how native species survive in these conditions, in order to understand how human impacts on springs change nearshore ecosystems. 

The researchers are excited to continue this work to better understand how native spring-associated limu tolerate spring conditions and what conditions they need to survive. We are also excited to link this work to watershed management strategies, especially how the new traits explain invasive algal blooms at nutrient polluted groundwater springs.

A boil of water at Waiʻalae ʻIki occurs where the submarine groundwater discharge spring enters the nearshore ecosystem. This brackish, nutrient rich water flows with enough force to cause a boil on the surface of the ocean.
A boil of water at Waiʻalae ʻIki occurs where the submarine groundwater discharge spring enters the nearshore ecosystem. This brackish, nutrient rich water flows with enough force to cause a boil on the surface of the ocean. 
Author Veronica Gibson holds up a handful of gorilla ogo and spiny seaweed above the nearshore submarine groundwater spring at Waiʻalae ʻIki.

Author Veronica Gibson holds up a handful of gorilla ogo and spiny seaweed above the nearshore submarine groundwater spring at Waiʻalae ʻIki.

Study species: a. Gorilla ogo, Gracilaria salicornia, b. Spiny seaweed, Acanthophora spicifera, c. Gracilaria perplexa, d. Limu maneʻoneʻo, Laurencia dendroidea. All scale bars are 1cm.

Study species: a. Gorilla ogo, Gracilaria salicornia, b. Spiny seaweed, Acanthophora spicifera, c. Gracilaria perplexa, d. Limu maneʻoneʻo, Laurencia dendroidea. All scale bars are 1cm. 

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Freshwater and Marine Ecology
Life Sciences > Biological Sciences > Ecology > Freshwater and Marine Ecology
Freshwater and Marine Ecology
Physical Sciences > Earth and Environmental Sciences > Earth Sciences > Biogeosciences > Freshwater and Marine Ecology
Plant Physiology
Life Sciences > Biological Sciences > Physiology > Plant Physiology

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