DOI: 10.5061/dryad.3j9kd51qz
title: Top-down vs. bottom-up: Grazing and upwelling regime alter patterns of primary productivity in a warm-temperate system

description: Community structure is driven by biological interactions and physical
 processes that can vary across environmental gradients and spatial scales.
 Early ecological models focused on the role of resource availability (i.e.
 bottom-up effects), predicting that the strength of top-down control
 varied along gradients of primary productivity and that local species
 interactions determined community structure. However, the role of regional
 scale oceanographic processes in determining species interactions and
 community structure is now widely recognized, with bottom-up effects such
 as coastal upwelling driving regional scale patterns of resource
 availability. Such nutrient subsidies can significantly alter primary
 production and drive changes in algae-herbivore interactions in rocky
 intertidal habitats. However, despite the potential for upwelling to alter
 these interactions, studies investigating the effects of upwelling and
 grazing pressure are scarce, particularly for warm-temperate systems, and
 generally cover narrow geographical ranges. Using in-situ herbivore
 exclusion experiments replicated across multiple upwelling regimes, we
 investigated the effects of both grazing pressure and upwelling, as well
 as their interactions, on the sessile invertebrate community and primary
 production of macroalgal communities in a warm-temperate system.
 Invertebrate cover remained consistently low at upwelling sites and was
 reduced at non-upwelling sites when grazers were excluded. Macroalgal
 cover was greater at upwelling sites when grazers were excluded and there
 was a strong effect of succession throughout the experimental period.
 Grazing pressure was greater at upwelling sites, particularly during
 winter months. There was a non-significant trend towards greater grazing
 pressure on early than later successional stages. Our results show that
 the positive impacts of bottom-up effects of nutrient supply on algal
 production do not overwhelm top-down control in this warm-temperate
 system. We speculate that global increases in air and sea-surface
 temperatures in warm-temperate systems will promote top-down effects in
 upwelling regions by increasing herbivore metabolic and growth rates.
description: <em>Experimental
 design</em>. At each
 site, 15 experimental plots were established in 5 blocks around the
 mid-shore level across approximately 100 metres of shoreline, with a
 minimum of 2 metres between blocks. Each block contained 1 replicate from
 each experimental treatment. The experiment had a fully factorial design
 with two factors: 1) upwelling regime (fixed, two levels: upwelling and
 non-upwelling) and 2) grazing pressure (fixed, three levels: total
 exclusion, partial exclusion, total access). Grazer exclusion plots were
 established by enclosing plots with fences (35 x 35 cm, 10 cm high)
 constructed from stainless steel mesh (0.9 mm wire diameter, 5.16 mm
 aperture) fixed to the substratum with screws and washers. Fences were
 chosen instead of enclosed cages with lids to prevent light limitation
 (Appendix S1: Section S2). This allowed the exclusion of all benthic
 grazers from the fenced  plots. To test for experimental artefacts
 affecting on macroalgal production as a result of the presence of the
 fences, procedural controls were established using fences around half of
 the plot in an L-shape to allow partial grazer access. Experimental
 controls were marked at the corner with screws only, allowing complete
 grazer access. Fences were cleaned of any epibiota during every sampling
 period. Every experimental treatment was replicated five times at each
 site but adverse weather and sea conditions meant some cages were lost and
 sample sizes varied from 3-5 replicates per treatment per site. Prior to
 the beginning of the experiment, all experimental and control plots were
 scraped clear to ensure all living organisms were removed and algal and
 sessile invertebrate cover was 0 %. Plots were left for a 4-week period
 before surveys began. The experiment started in January 2021 and was
 surveyed monthly for 12 months. <em>Data collection</em>. To quantify benthic grazing pressure, grazer
 density was recorded monthly at each site. Ten 25 x 25 cm quadrats were
 placed haphazardly throughout the mid-shore level and the identity
 (species level) and density of grazers were recorded. As grazer size has
 been shown to be an important factor in determining the outcome of
 algal-grazer interactions, grazers were classified into meso- and
 macro-grazers. On South African shores, dominant macro-grazers in the
 mid-low intertidal, including the gardening limpet <em>Scutellastra
 longicosta</em> and the limpet <em>Cymbula oculus,</em>
 are large individuals, &gt; 20 mm, and well adapted to wave-exposed
 conditions (Díaz &
 McQuaid 2011, Branch & Odendall 2003; Nakin & McQuaid
 2014). Meso-grazers are
 also abundant at the mid-shore level and include pulmonate limpets such as
 <em>Siphonaria </em>spp. and gastropods including
 <em>Oxystele </em>spp. approximately 15-20 mm in size
 (Whittington-Jones, 1997;
 Hodgson 1999, Díaz & McQuaid 2011). As these were the dominant grazer species
 identified in the current study, a 20 mm cut-off between grazer size
 classes was chosen to distinguish between meso- and macro-grazers.   
 To quantify the effects of grazing pressure and
 upwelling regime on the colonisation of sessile invertebrates, percentage
 cover for each sessile invertebrate species in each plot was estimated
 monthly using the point intercept method with a 25 cm x 25 cm quadrat with
 36 intersections, positioned carefully within each cage to avoid sampling
 edge effects. Although sedentary species such as mussels were present
 during the experiment, abundances were very low (&gt; 1 individual per
 plot) and invertebrate communities were dominated by sessile invertebrates
 (predominantly barnacles). We therefore refer to sessile, not sedentary,
 invertebrate communities. A similar method was used to test for the
 effects of grazing pressure and upwelling regime on macroalgal percentage
 cover (as a proxy for macroalgal production). Sessile invertebrates and
 macroalgae were identified to species level where possible and species
 present within the quadrat but not occurring directly beneath an
 intersection were assigned a percentage cover value of 1 %
 (O’Connor & Crowe
 2005). Total percent cover
 values of macroalgae sometimes exceeded 100 % owing to the multi-story
 nature of macroalgal communities. Grazers found in grazer-exclusion plots
 during monthly surveys were removed. At the end of the experiment, biomass
 was destructively removed from all plots, separated into species and dried
 at 60°C until constant weight.
description: # Data from: Top-down vs. bottom-up: Grazing and upwelling regime alter
 patterns of primary productivity in a warm-temperate system
 [https://doi.org/10.5061/dryad.3j9kd51qz](https://doi.org/10.5061/dryad.3j9kd51qz) This data pertains to a manipulative field experiment in which in-situ stainless steel fences were used to test for the effects of grazing pressure and upwelling regime on the sessile invertebrate community and macroalgal productivity of an intertidal rocky shore in a warm-temperate system. This dataset contains grazer abundance, invertebrate cover and macroalgal cover and biomass data collected at two upwelling and two non-upwelling sites in the presence and absence of grazers on the southeast coast of South Africa during monthly quadrat surveys between January 2021 and December 2021. Grazer access was manipulated using stainless steel fences with three treatments: full fence (no access), half fence (partial access- procedural control) and controls (no fence- full access). Treatments were surveyed monthly using a 25 x 25 cm quadrat in which all grazers within the quadrat were identified to species level and quantified. Algae and invertebrate cover were also identified to species level and quantified using the point intercept method. At the end of the experiment, all macroalgal biomass was destructively sampled within each treatment, separated into species and dried at 60 degrees celsius. This dataset also contains monthly grazer and invertebrate abundance surveys at two upwelling and two non-upwelling sites on the southeast coast of South Africa in which 10 randomly placed 25 x 25 cm quadrats were surveyed for grazer abundance, identified to species level, and invertebrate cover, quantified using the point intercept method. ## Description of the data and file structure This dataset may be used in the case of a meta analysis looking at the effects of grazing pressure and upwelling regime (nutrients) on the productivity and species composition of intertidal rocky shores. The data provided are separated into data type, with each sheet corresponding to the following dataset: * Survey\_Grazer\_Count: Density of grazer species identified during monthly quadrat surveys using a 25 x 25 cm quadrat at two upwelling and two non-upwelling sites on the southeast coast of South Africa between January 2021-December 2021. * Survey\_Invert\_Count: Percentage cover of invertebrate species identified during monthly surveys using the point intercept method with a 25 x 25 cm quadrat at two upwelling and two non-upwelling sites on the southeast coast of South Africa between January 2021-December 2021. * Cage\_Algae\_%Cover: Percentage cover of algal species within grazer exclusion treatments (full fence, half fence, control) identified during monthly surveys using the point intercept method with a 25 x 25 cm quadrat at two upwelling and two non-upwelling sites on the southeast coast of South Africa between January 2021-December 2021. * Cage\_Grazer\_Count: Density of grazer species within grazer exclusion treatments (full fence, half fence, control) identified during monthly surveys with a 25 x 25 cm quadrat at two upwelling and two non-upwelling sites on the southeast coast of South Africa between January 2021-December 2021. * Cage\_Invert\_%Cover: Percentage cover of invertebrate species within grazer exclusion treatments (full fence, half fence, control) identified during monthly surveys using the point intercept method with a 25 x 25 cm quadrat at two upwelling and two non-upwelling sites on the southeast coast of South Africa between January 2021-December 2021. * Cage\_Algae\_Succession: Percentage cover of algal species classified as early (first three months of the experimental period) and late (last three months of experimental period) within grazer exclusion treatments (full fence, half fence, control) quantified using the point intercept method with a 25 x 25 cm quadrat at two upwelling and two non-upwelling sites on the southeast coast of South Africa between January 2021-December 2021. * Cage\_Algae\_Biomass: Total biomass (g dry weight) of all algal species within grazer exclusion treatments (full cage, half cage, control) quantified at the end of the experimental period at two upwelling and two non-upwelling sites on the southeast coast of South Africa between January 2021-December 2021. The dataset contains the following abbreviations: SP: sampling period * 1- January 2021 * 2- February 2021 * 3- March 2021 * 4-April 2021 * 5- May 2021 * 6- June 2021 * 7- July 2021 * 8- August 2021 * 9- September 2021 * 10- October 2021 * 11- November 2021 * 12- December 2021 Location * JB- Jeffreys Bay * CSF- Cape St. Francis * EL- East London * PA- Port Alfred Nutrients: upwelling regime * NU- non-upwelling * U- upwelling Size: size classification of grazers * Macro- macrograzers &gt; 20 mm * Micro- micrograzers &lt; 20 mm Succession: classification of algal species into 'early' or 'late' colonisers * E- early * L- late Treatment: grazer access/exclusion treatments created using stainless steel cages * F- full fence (total grazer exclusion) * H- half fence (partial grazer exclusion, procedural control) * C- control (total grazer access, no fence) The following headings are used to describe columns in the dataset: * Quadrat- quadrat number for quadrat surveys at each site during each sampling * Block- treatments were grouped together into five blocks at each site, with each block containing one full cage, one half cage and a control * Count- density of each grazer species within each quadrat * Cover- % cover of algae and invertebrate species within each quadrat quantified using the point intercept method * Biomass- total biomass (g of dry weight) of all algal species within each treatment at the end of the experimental period * Species- species identified within each quadrat Missing data: due to bad weather and lost cages, some datasets do not contain 5 replicates of each treatment for each site during each sampling period. ## Sharing/Access information ## Code/Software