Nitrogen stable isotopes (δ¹⁵N) and tissue nitrogen in shallow‐water and mesophotic macroalgae differ between the Main Hawaiian Islands and the Northwestern Hawaiian Islands

Abstract

The Hawaiian Archipelago stretches 2500 km from the Main to the Northwestern Hawaiian Islands, represents a complex gradient of oceanographic and anthropogenic drivers, and has a high abundance and diversity of native and invasive macroalgae. These photosynthetic organisms occur in intertidal to mesophotic (30–150+ m) depths and absorb nitrogen with limited fractionation associated with their physiology and source. Our goal was to examine nitrogen dynamics from shallow to mesophotic reefs using compositional patterns of two well‐characterized macroalgal tissue parameters: stable isotope ratio of nitrogen and tissue nitrogen content. We collected 813 macroalgal samples from 13 islands/atolls between 0 and 117 m depths. Within the Main Hawaiian Islands, macroalgal tissue stable N isotope ratios were higher in mesophotic depths; N content was higher in shallow depths. However, within the Northwestern Hawaiian Islands, no differences in stable N isotope ratios and N content were found between shallow and mesophotic depths. Regionally, stable N isotope ratios varied along a gradient of anthropogenic and oceanographic processes (in Main and Northwestern Hawaiian Islands, respectively), while N content reflected elevated nitrogen in the Main compared with the Northwestern Hawaiian Islands. Additionally, the invasive macroalga Avrainvillea lacerata had significantly higher N content than co‐occurring native bryopsidalean macroalgae at similar depths, and may be reshaping nutrient dynamics from shallow to mesophotic depths in the Main Hawaiian Islands. Nitrogen dynamics at mesophotic depths may be influenced by nearshore anthropogenically derived nitrogen via submarine groundwater discharge and/or inputs from deeper water within the Main Hawaiian Islands.

Mesophotic corals in Hawai‘i maintain autotrophy to survive low-light conditions

“…This degradation probably contributed to δ 15 N increases of 1‰ in coral skeletons observed from 5 to 105 m in Palau, mirroring increases in the δ 15 N of suspended POM [49]. Around Maui, increases in mean macroalgal δ 15 N from shallow (0–30 m; 2.4‰) to mesophotic depths (30–117 m; 2.9‰) were attributed to submarine groundwater discharge potentially linked to anthropogenic nutrient pollution [30]. All such environmental increases in source δ 15 N and hypothesized increases in heterotrophy through the mesophotic zone [27] should increase host δ 15 N alone or increase both host and symbiont δ 15 N across depth.…”

“…This current can increase mixing of zooplankton and detritus across mesophotic depths [27,51], potentially increasing heterotrophy relative to our winter collections. Differences among submersible collection sites and microhabitats in the ‘Au‘au Channel between years may have compounded trophic differences between these two distinct collections due to spatial variations in submarine groundwater discharge [30] and thermal variability [16].…”

“…However, the extent that mesophotic reef corals rely on heterotrophy versus autotrophy with increasing depth is debated [16,27,28]. Recent evidence of anthropogenic impacts on mesophotic reefs, including elevated temperatures [29] and nutrient pollution [30], emphasize the need to conserve these vast ecosystems. Effective management strategies will require a better understanding of energy uptake, allocation and niche differentiation in mesophotic corals.…”

Unveiling mesophotic diversity in Hawai‘i: two new species in the genera <italic>Halopeltis</italic> and <italic>Leptofauchea</italic> (Rhodymeniales, Rhodophyta)

“…Influxes of cold water in the Hawaiian mesophotic provide suitable habitat for algae typically found in temperate zones (Abbott and Huisman 2003), resulting in certain species and genera being found only in the mesophotic and absent from the better-studied adjacent shallow waters (Cabrera et al 2022). In the Main Hawaiian Islands, mesophotic macroalgae may also be influenced by nearshore anthropogenically derived nitrogen via submarine groundwater discharge (Strait et al 2022).…”