Autochthonous carbon loading of macroalgae stimulates benthic biological nitrogen fixation rates in shallow coastal marine sediments

Macroalgae, commonly known as seaweed, are foundational species in coastal ecosystems and contribute significantly to coastal primary production globally. However, the impact of macroalgal decomposition on benthic biological nitrogen fixation (BNF) after deposition to the seafloor remains largely unexplored. In this study, we measure BNF rates at three different sites at the Big Fisherman's Cove on Santa Catalina Island, CA, USA, which is representative of globally distributed rocky bottom macroalgal habitats. Unamended BNF rates varied among sites (0.001-0.05 nmol N g-1 h -1) and were generally within the lower end of previously reported ranges. We hypothesized that the differences in BNF between sites were linked to the availability of organic matter. Indeed, additions of glucose, a labile carbon source, resulted in 2-3 orders of magnitude stimulation of BNF rates in bottle incubations of sediment from all sites. To assess the impact of complex, autochthonous organic matter, we simulated macroalgal deposition and remineralization with additions of brown (i.e., Macrocystis pyrifera and Dictyopteris), green (i.e., Codium fragile), and red (i.e., Asparagopsis taxiformis) macroalgae. While brown and green macroalgal amendments resulted in 53- to 520-fold stimulation of BNF rates-comparable to the labile carbon addition-red alga was found to significantly inhibit BNF rates. Finally, we employed nifH sequencing to characterize the diazotrophic community associated with macroalgal decomposition. We observed a distinct community shift in potential diazotrophs from primarily Gammaproteobacteria in the early stages of remineralization to a community dominated by Deltaproteobacteria (e.g., sulfate reducers), Bacteroidia, and Spirochaeta toward the latter phase of decomposition of brown, green, and red macroalgae. Notably, the nifH-containing community associated with red macroalgal detritus was distinct from that of brown and green macroalgae. Our study suggests coastal benthic diazotrophs are limited by organic carbon and demonstrates a significant and phylum-specific effect of macroalgal loading on benthic microbial communities.

Review and guide to the isopods (Crustacea, Isopoda) of littoral and sublittoral marine habitats in the Southern California Bight

The isopod crustaceans reported from or expected to occur in littoral and sublittoral marine habitats of the Southern California Bight (SCB) in the northeastern Pacific Ocean are reviewed. A total of 190 species, representing 105 genera in 42 families and six suborders are covered. Approximately 84% of these isopods represent described species with the remaining 16% comprising well-documented "provisional" but undescribed species. Cymothoida and Asellota are the most diverse of the six suborders, accounting for ca. 36% and 29% of the species, respectively. Valvifera and Sphaeromatidea are the next most speciose suborders with between 13-15% of the species each, while the suborder Limnorioidea represents fewer than 2% of the SCB isopod fauna. Finally, the mostly terrestrial suborder Oniscidea accounts for ca. 5% of the species treated herein, each which occurs at or above the high tide mark in intertidal habitats. A key to the suborders and superfamilies is presented followed by nine keys to the SCB species within each of the resultant groups. Figures are provided for most species. Bathymetric range, geographic distribution, type locality, habitat, body size, and a comprehensive list of references are included for most species.

A decade of invertebrate recruitment at Santa Catalina Island, California

Marine fouling communities have long provided model systems for studying the ecology of community development, and settlement plates are the tool of choice for this purpose. Decades of plate deployments provide a baseline against which present-day trends can be interpreted, with one classic trend being the ultimate dominance of plates by colonial and encrusting taxa. Here we report the results of annual deployments of settlement plates from 2010 to 2021 in the shallow sub-tidal of southern California, where the recruitment of invertebrates and algae was recorded photographically, and resolved to functional group (solitary, encrusting, and arborescent) and the lowest taxon possible. The communities on these plates differed among years, with trends in abundances varying by functional group and taxon; solitary taxa consistently were abundant, but encrusting taxa declined in abundance. Seawater temperature and the subsurface concentration of chlorophyll a differed among years, and there was a weak inverse association between temperature and the abundances of encrusting taxa. Long-term increases in seawater temperature therefore could serve as a mechanism causing fouling communities to change. Because of the prominence of encrusting taxa in fouling communities, the shifts in abundance of this functional group reported here may portend ecologically significant changes in fouling communities exposed to warmer seawater because of an alleviation of competition for a classically limiting resource (i.e., space).