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Seasonal upwelling reduces herbivore control of tropical rocky intertidal algal communities

Citation

Sellers, Andrew et al. (2021), Seasonal upwelling reduces herbivore control of tropical rocky intertidal algal communities, Dryad, Dataset, https://doi.org/10.5061/dryad.brv15dv7n

Abstract

Communities are shaped by a variety of ecological and environmental processes, each acting at different spatial scales. Seminal research on rocky shores highlighted the effects of consumers as local determinants of primary productivity and community assembly. However, it is now clear that the species interactions shaping communities at local scales are themselves regulated by large-scale oceanographic processes that generate regional variation in resource availability. Upwelling events deliver nutrient-rich water to coastal ecosystems, influencing primary productivity and algal-herbivore interactions. Despite the potential for upwelling to alter top-down control by herbivores, we know relatively little about the coupling between oceanographic processes and herbivory on tropical rocky shores, where herbivore effects on producers are considered to be strong. By replicating seasonal molluscan herbivore exclusion experiments across three regions exposed to varying intensity of seasonal upwelling, separated by hundreds of kilometers along Panama’s Pacific coast, we examine large-scale environmental determinants of consumer effects and community structure on tropical rocky shores. At sites experiencing seasonal upwelling, grazers strongly limited macroalgal cover when upwelling was absent, leading to dominance by crustose algae. As nutrients increased and surface water cooled during upwelling events, increases in primary productivity temporarily weakened herbivory, allowing foliose, turf and filamentous algae to replace crusts. Meanwhile, grazer effects were persistently strong at sites without seasonal upwelling. Our results confirm that herbivores are key determinants of tropical algal cover, however, our focus on regional oceanographic conditions revealed that bottom-up processes regulate top-down control on tropical shorelines. This study expands on the extensive body of work highlighting the influence of upwelling on local ecological processes by demonstrating that nutrient subsidies delivered by upwelling events can weaken herbivory in tropical rocky shores.

Methods

The datasets included in this submission are raw data collected from a molluscan herbivore exclusion experiment performed in intertidal rocky shores in the Pacific coast of Panama. The experiments were replicated across 6 sites in three regions, and the goal was to examine how seasonal and regional differences in oceanographic conditions influence herbivory in tropical shores. The first data file are raw data of changes in benthic cover in different treatments and sites throughout the duration of each seasonal experiment (experimentResults.csv). The other three data files (grazerDayCounts.csv, grazerNightCounts.csv, and surveyResults.csv) are surveys of molluscan grazers performed inside and outside the experimental plots. The methodology for the experiments and surveys are outlined below.

Herbivore exclusion experiment

To examine temporal and spatial variation in algal cover and herbivore effects we performed a three-month grazer exclusion experiment, replicated at the six study sites during two wet (August-November, 2016 and 2017) and two dry seasons (January-April, 2017 and 2018). We initiated each trial by clearing the benthic biota from twenty-one 25 x 25 cm plots using metal scrapers and wire brushes, then burned the substrate with a propane torch. This allowed us to examine grazer effects during the initial stages of community assembly, when effects of mollusks on algal cover are strongest. It also enabled us to compare results across seasons and regions, without potential artifacts generated by different initial communities.

We grouped the plots at each site into 7 blocks of 3 plots, and randomly assigned one of three treatments to each plot. The grazer exclusions deterred molluscan grazers (henceforth grazers) with a barrier of copper strips (Corry’s Slug and Snail Copper Tape Barrier, Atlanta, GA, USA) and synthetic grass cemented to the rock around the plot’s perimeter with Z-Spar epoxy compound. Open controls allowed access to grazers and corners were marked with a dab of epoxy. Procedural controls tested for potential artifacts of our exclusion method, and consisted of discontinuous copper and grass barriers epoxied around 50% of the perimeter. We included a fourth unmanipulated and uncleared treatment to each block to track seasonal changes in the existing sessile assemblages. Procedural and uncleared controls were not included in our first trial: wet-season 2016.

Every two to three weeks, we characterized benthic cover, counted mobile consumers inside all plots, and removed any that had infiltrated exclusion plots. Grazer activity increased during nighttime low tides, so we counted consumers at night in September 2016 and February 2017 in three accessible sites. To quantify sessile growth inside plots we placed a metal grid with 100 random points and identified the organism under each point to the lowest taxonomic level possible. Algae were grouped into functional groups according to common classifications: foliose, corticated, crustose, filamentous, and articulated coralline. We collected sessile growth inside each plot when the trials ended to measure algal biomass, however, we did not use those data because it was difficult to remove barnacle fragments prior to weighing samples, making the data unreliable. Thus, our analysis of algal abundance focused on percent cover data.

Molluscan grazer surveys

Differences in grazer density among sites can drive spatial variation in herbivory, thus, in addition to monitoring the abundance of grazers in our plots, we also estimated herbivore density at each site by counting all individuals inside fifteen to twenty 0.25 m2 quadrats haphazardly placed in the low intertidal. Surveys took place during daytime low tides between April and June 2016 (wet season), and between January and March 2018 (dry season).

Usage Notes

Column headers for experimentResults.csv, grazerDayCount.csv, and grazerNightCount.csv

season: nu16 (wet-season 2016), u17 (dry-season 2017), nu17 (wet-season 2017), u18 (dry-season 2018). Wet-seasons correspond to non-upwelling (nu) periods, while dry-seasons are upwelling (u) periods.

region: GC (Gulf of Chiriqui), AP (Azuero Peninsula), GP (Gulf of Panama)

site: IC (Isla Catalina), PB (Punta Brava), AC (Achotines), DE (Destiladeros), PS (Playa Serena), CU (Culebra)

check: The experimental trials lasted 3 months, and were surveyed monthly - 0 (plot check after 1 month), 1 (plot-check after 2 months), 3 (plot-check after 3 months).

date: date when plot was surveyed ('checked').

block: code corresponding to each individual experimental block of treatments

treat: experimental treatments (C= open control), (PC= procedural control), (UC= unmanipulated control), E (molluscan exclusion).

 

Funding

Smithsonian Tropical Research Institute

McGill University