Coastal ocean acidification and nitrogen loading facilitate invasions of the non-indigenous red macroalga, Dasysiphonia japonica

Coastal ecosystems are prone to multiple anthropogenic and natural stressors including eutrophication, acidification, and invasive species. While the growth of some macroalgae can be promoted by excessive nutrient loading and/or elevated pCO(2), responses differ among species and ecosystems. Native to the western Pacific Ocean, the filamentous, turf-forming rhodophyte, Dasysiphonia japonica, appeared in estuaries of the northeastern Atlantic Ocean during the 1980s and the northwestern Atlantic Ocean during the late 2000s. Here, we report on the southernmost expansion of the D. japonica in North America and the effects of elevated nutrients and elevated pCO(2) on the growth of D. japonica over an annual cycle in Long Island, New York, USA. Growth limitation of the macroalga varied seasonally. During winter and spring, when water temperatures were < 15 degrees C, growth was significantly enhanced by elevated pCO(2) (p < 0.05). During summer and fall, when the water temperature was 15-24 degrees C, growth was significantly higher under elevated nutrient treatments (p < 0.05). When temperatures reached 28 degrees C, the macroalga grew poorly and was unaffected by nutrients or pCO(2). The delta C-13 content of regional populations of D. japonica was -30 parts per thousand, indicating the macroalga is an obligate CO2-user. This result, coupled with significantly increased growth under elevated pCO(2) when temperatures were < 15 degrees C, indicates this macroalga is carbon-limited during colder months, when in situ pCO(2) was significantly lower in Long Island estuaries compared to warmer months when estuaries are enriched in metabolically derived CO2. The delta N-15 content of this macroalga (9 parts per thousand) indicated it utilized wastewater-derived N and its N limitation during warmer months coincided with lower concentrations of dissolved inorganic N in the water column. Given the stimulatory effect of nutrients on this macroalga and that eutrophication can promote seasonally elevated pCO(2), this study suggests that eutrophic estuaries subject to peak annual temperatures < 28 degrees C may be particularly vulnerable to future invasions of D. japonica as ocean acidification intensifies. Conversely, nutrient reductions would serve as a management approach that would make coastal regions more resilient to invasions by this macroalga.

Automated summary

Coastal ecosystems are susceptible to various stressors, including nutrient loading, acidification, and invasions. The growth of the filamentous red algae Dasysiphonia japonica is influenced by nutrient availability and pCO(2) levels, with seasonal variations in growth responses. This study in Long Island, New York, found that the growth of D. japonica was carbon-limited during colder months and nitrogen-limited during warmer months, with implications for future invasions and ecosystem management.