Dataset: Water column nutrient data from a 4 year experiment (2018-2022) manipulating consumer pressure and nutrient availability on a coral reef in Moorea, French Polynesia

Experimental Setup

In June 2018, we established a factorial experiment manipulating consumer pressure and nutrient availability in areas that mimicked disturbance from a cyclone. At 12m depth on the north shore forereef of Moorea, French Polynesia (17.47° S, 149.82° W) we established 16 ~30m² plots. Within each plot, we then nested 4 different consumer exclosures (~1.25 m² each) with different size openings that allowed different size fishes (herbivores and corallivores, hereafter referred to as ‘consumers’) access to the benthos . The exclosure frames consisted of 0.5cm stainless steel all-thread drilled into the reef matrix and epoxied into place. These frames were then wrapped with plastic-coated, galvanized wire to create the following levels of consumer pressure: 1) Very Low (2.5cm × 2.5cm openings); 2) Low (5cm × 5cm openings); 3) Medium (7.5cm × 7.5cm openings); 4) High (4 sides of 2.5cm × 2.5cm openings but no top). Others have used a similar design to create a gradient of consumer pressure to mimic the effects of different levels of fishing (Holbrook et al. 2016, Schmitt et al. 2019). We included sides, but not tops, on the High consumer pressure treatment to control for potential artifacts on water flow, although we have shown these are minimal (Zaneveld et al. 2016). Exclosures were scrubbed every 12-16 weeks to remove fouling organisms.

We also included a second treatment of Ambient or Enriched nutrient conditons with each plot (which included four nested consumer exclosures) was then assigned to either Ambient or Enriched nutrient conditions. Thus, each combination of consumer pressure (Very Low, Low, Medium, High) and nutrients (Ambient or Enriched) had n=4 for replication. For the enrichment we placed 175 g of Osmocote^® (19-6-12, N-P-K) slow-release garden fertilizer into 5 cm diameter PVC tubes with 10, 1 cm holes drilled into them. These tubes were wrapped in fine plastic mesh to retain the fertilizer. This method is similar to our previous work (e.g., Zaneveld et al. 2016). PVC enrichment tubes were attached to the corners of each exclosure and onto a piece of stainless steel all-thread in the center of each plot (5 enrichment tubes per exclosure). We replaced enrichment tubes every 12-16 weeks except for two periods during the COVID-19 pandemic when travel to Moorea was not possible and enrichment tubes were deployed for longer than usual before replacement (deployed from 01-30-2020 to 08-31-2020, and from 08-31-2020 to 02-08-2021). We analyzed water samples from the experimental plots to evaluate the effect of nutrient enrichment.

Data Collection

To evaluate the effectiveness of our nutrient treatment, we collected water samples using 50-ml syringes ~3cm above the benthos in each of our High consumer pressure exclosures only (i.e., our open 1.25m² exclosures). We collected water samples in 2021 and 2022 across a range of days after we swapped the nutrient diffusers (4-158 days). We filtered samples (GF/F) into falcon tubes, placed on ice and once back in the lab frozen until analysis. We analyzed inorganic nitrogen (nitrite + nitrate) and soluble reactive phosphorus concentrations using an autoanalyzer at the Marine Science Institute Analytical Laboratory at the University of California, Santa Barbara.