The physical and physiological impacts of the invasive red macroalga Gracilaria salicornia Dawson to coral reef habitat quality and coral health

Abstract

Gracilaria salicornia Dawson is a successful and prolific red invasive alga, which grows in large stands of biomass that accumulate mats as large as 5 kg m-2 and effectively smothers native coral reef organisms. This research aims to understand the reproductive biology and ecology of the indigenous Hawaiian coral Porites Synarӕa hawaiiensis. P. hawaiiensis is a brooding coral, capable of reproducing any day of the year, producing non-swimming motile larvae that show a strong affinity for settlement near crustose coralline algae, with low variability and maximal settlement within two days. This research also aimed to answer the central question of, how does overgrowth by invasive macroalgal species affect benthic habitat quality and coral biology on coral reefs? Data from a series of manipulative studies are presented to investigate the impacts of algal mats to physical parameters on coral reefs such as: sediment, irradiance and dissolved gas variability as well as: coral recruitment, and coral health. The algal mats attenuated irradiance by as much as 99%, effectively doubled the amount of sediment deposited to the reef area below, imposed nightly periods of hypoxia lasting from 30 minutes to 120 minutes, imposed periods of high carbon dioxide concentrations and lowered pH by as much as a tenth of a pH point as a function of algal and community respiration. When the overgrowth of the coral Porites compressa by algal mats was simulated, the coral bleached within two weeks and the algal symbionts increased pigment levels to enhance photosynthesis. When algal management was simulated by removing the algae, pigments in the algal symbionts returned to normal. Oxidative stress through DNA damage and elevated incidences of anaerobic respiration through the accumulation of L-lactate was not observed, suggesting that this species of coral was resilient enough to defend and recover from this stress. These findings suggest that management control efforts to remove invasive algal mats are likely to be successful in restoring the health and recovery of this species in Hawaii. Physiological metrics of the efficacy of management action are powerful tools that are available and can give shortterm information and allow for adaptive management.