Influences of grazing and nutrients on benthic algae in Hawaiian anchialine pools

Other Team Members (external): 
Robert Nishimoto


West Hawaii Coast HI
United States




Anchialine pools are land-locked brackish water habitats located near the coast that are heavily influenced by ocean tides and concurrent fresh groundwater influx through subterranean connections. These physically dynamic pools are inhabited by many rare species that are found exclusively in these habitats. One such species is an endemic herbivorous shrimp, Halocaridina rubra, known locally as 'opae 'ula, which is considered to play a vital role in maintaining the bio-ecological integrity of these pools.

Unfortunately, a vast majority of Hawaiian anchialine habitats have been in some way lost or degraded due to a rapid and an incessant increase in human development. Some of the suspected impacts of development are increased levels of nutrients that enter these pools, and altered behavior, or even extirpation of the endemic 'opae 'ula.  This, in turn, could affect the algae growing in these pools and therefore affect the overall health and function of these anchialine habitats.

The primary objective of this study is to investigate exactly how elevated nutrients and alterations to grazing by 'opae 'ula affect algal growth, abundance and composition.  Terra-cotta plates were deployed for one month in 11 anchialine pools along the West Hawaii coast to collect algae samples. Experimental treatments included enriching some plates with different nutrients (i.e., Nitrogen, Phosphorus, Nitrogen+Phosphorus), as well as allowing some plates to be grazed by 'opae 'ula and others excluded from grazers. The plates were collected after one month and all the algae that grew on each plate was analyzed for biomass, Chlorophyll a, composition and other physiological properties. We were then able to compare these measurements across the different experimental treatments to determine the separate and combined effects of nutrients and grazing on algae. The results from this project will enhance our fundamental knowledge of how Hawaiian anchialine ecosystems function and how they respond to multiple stressors.


Recent Achievements


Nutrient additions of N, P or N+P did not have any significant effect on algal growth or biomass. Moreover, measurements of algal production (Chlorophyll a) and biomass were inversely correlated to the ambient nutrient concentrations in each pool- higher ambient nutrient concentrations resulted in lower algal production. This is in contrast to what we expected. Therefore, elevated levels of nutrients may be having some sort of inhibiting effect on algal production, or changing the types of algae that are growing. However, further investigation is warranted to verify these phenomena.


Our data has also shown that grazing by 'opae 'ula has the greatest influence on algal biomass, as significant reductions in algal biomass resulted from grazing. Although this may sound like a negative impact, excessive algal growth and biomass are detrimental to the benthic habitat, water circulation, water quality and the overall health of the anchialine ecosystem. Anecdotal evidence has suggested that grazing also influences the general types of algae that grow. Grazing by ‘opae ‘ula therefore serves as a regulatory mechanism of algae and is thus essential to maintaining Hawaiian anchialine ecosystem function and quality. This is especially true in the occurrence of increased nutrient input.



Future Work

We are currently working to identify the algae samples into taxonomic groups (i.e., species, genus, family etc.) and functional groups (i.e., diatoms, filamentous algae) to determine if there are differences in algal composition between both grazing and nutrient treatments. These results will verify if 'opae 'ula are selectively grazing on certain types of algae and if both grazing and nutrients influence the species composition of algae. Such information has not yet been obtained from Hawaiian anchialine pools.

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Collaborators & Partners

Division of Aquatic Resources (DAR) - DLNR
DAR has facilitated field sampling, finding sample sites, and advised in anchialine survey methodolo
  • Troy Sakihara
  • Robert Nishimoto
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Institute of Pacific Islands Forestry - US Forest Service (USFS)
Rich has provided continual guidance, support and advising towards the experimental design, methodol
  • Richard Mackenzie
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University of Hawaii Hilo (UHH)
Bruce has provided continual guidance, support and advising on the experimental design, field sampli
  • Jim Beets
  • Bruce Dudley
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