DOI: 10.5061/dryad.brv15dv7n
title: Seasonal upwelling reduces herbivore control of tropical rocky intertidal algal communities

description: 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.
description: 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. <i>Herbivore exclusion
 experiment</i> 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. <i>Molluscan grazer
 surveys</i> 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 m<sup>2</sup> 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).
description: 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).