Image analysis of epiphyte-seagrass dynamics on Thalassia testudinum from different environmental conditions

Seagrasses are globally threatened due to increasing environmental stressors in coastal ecosystems. Excessive accumulation of algal epiphytes is suggested to be harmful to seagrass. The biomass and morphological measures widely used in understanding seagrass-epiphyte relationships provide limited insight into the dynamics of epiphyte colonization relative to leaf growth and senescence. Color scanning and image analysis methods were developed to characterize epiphyte accumulation with seagrass growth. Thalassia testudinum collected monthly or bimonthly from July 2019 to April 2020 near Redfish Bay, Texas, was analyzed through traditional and image-based measures. Spectral Angle Mapper (SAM) algorithms within ENVI Program distinguished the pixels of epiphyte-free leaf blades from many epiphytes. Unclassifiable pixels averaged < 5%. Classification accuracy was also evaluated by correlation of traditional biomass and morphology metrics vs. image-based metrics for seagrass (n = 2052) and epiphyte (n = 1822) collected across different seasons and environmental conditions. Image-derived leaf area strongly correlated with leaf biomass (R2 = 0.98, P < 0.0001) but linear regressions of epiphyte biomass vs. epiphyte area (pixels) (R2 = 0.61, P < 0.0001) and biomass ratio of epiphyte to seagrass vs. epiphyte coverage (epiphyte pixels/leaf pixels) (R2 = 0.51, P < 0.0001) were weaker. However, correlations greatly improved (R2 = 0.52 ~ 0.98, P < 0.0001), and the epiphyte accumulation presented linear or exponential patterns when parsed by season and environmentally different sites. The observation from both traditional and image-based metrics indicated that the seagrass-epiphyte relationship changed significantly across environmental context (P 0.05). The observation that the mean epiphyte coverage stayed relatively constant (maximum range of variation was about 15%) across seasons, but differed by vi site, suggests that leaf growth may be regulated to maintain the proportion of uncolonized leaf surface. The epiphyte accumulation relative to seagrass was greatest at low temperatures and at sites with elevated N:P ratio in sediment porewater. Image analysis may be insightful as an indicator of environmental change and suggests that epiphyte accumulation combines linear and exponential processes representing its colonization and growth. Future work will involve optimizing the spectral libraries to improve algal group classification to include diverse epiphytic community components.