Data for: Kelp forest loss and emergence of turf algae reshapes energy flow to predators in a rapidly warming ecosystem

Climate change is decimating habitat-forming species in ecosystems around the world. Yet the impacts of habitat loss on the energetics of the wider food web remain uncertain for many iconic ecosystems, including cold-water kelp forests. Here, we assessed how the loss of kelp forests and subsequent proliferation of low-lying turf algae in the Gulf of Maine has altered the trophic niches of, and energy acquired by, predatory reef fishes. Bulk tissue δ13C and δ15N analysis of fish muscle showed that fishes in kelp forests had larger trophic niches and greater interspecific niche separation than did fishes on turf reefs. Moreover, δ13C analysis of essential amino acids revealed that kelp-derived energy accounted for the majority of energy used by forest fishes (>50 % on average), whereas fishes on turf reefs compensated for kelp decline via greater reliance on a phytoplankton-based energy channel. Therefore, ecosystem state shifts to turf algae – now a global phenomenon – may have far-re...,
Section 1: Underwater surveys

Data1_macroalgae_biomass.csv
Data2_roving_fishes.csv
Macroalgae surveys.
During the summers of 2021 and 2022, we characterized macroalgae assemblages on subtidal rocky reefs via scuba (n = 11-16 sites per year, 3-5 per subregion). At each site, divers laid a 40 m transect along the isobath at 5-7 m depth, parallel to shore, then placed 1 m2 quadrats (n = 4-6 quadrats per site) at predetermined intervals along the transect. We collected all kelps (i.e., canopy-forming brown macroalgae in the order Laminariales) from the 1 m2 quadrat and all understory macroalgae (i.e., the diverse consortium of bladed, foliose, and filamentous seaweed taxa residing under the canopy) from a ¼ m2 area of the quadrat. In the lab, we sorted macroalgae by species, identified them to the lowest possible taxonomic level, dried them by spinning, and weighed each. We performed these surveys between July and mid-September (oceanographic summer) of both years.
Fish surveys.
To charac..., # Data for: Kelp forest loss and emergence of turf algae reshapes energy flow to predators in a rapidly warming ecosystem

Dataset DOI: [10.5061/dryad.bcc2fqzqz](10.5061/dryad.bcc2fqzqz)

## Description of the data and file structure

Our study took place in four subregions of the Maine coast that span a gradient of temperature and kelp forest habitat condition. At each study site, we conducted surveys to quantify available basal resources (macroalgae) and characterize the fish assemblage. We then collected representative primary producers and the two dominant fish species for stable isotope analysis. We analyzed these fish using both bulk tissue and compound-specific stable isotope analyses to document fish isotopic niches and energy use on kelp-dominated vs. red turf algae-dominated reefs.

## Files and variables

### File: Data1\_macroalgae\_biomass.csv

**Description:** This dataset contains measurements of seaweed biomass (i.e., kelp, red macroalgae, and other seaweeds) collected ...,