Frey DL and Gagnon P (2015), Thermal and hydrodynamic environments mediate i...

ECB-ART-43876

PLoS One 2015 Jan 01;103:e0118583. doi: 10.1371/journal.pone.0118583.

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Thermal and hydrodynamic environments mediate individual and aggregative feeding of a functionally important omnivore in reef communities.

Frey DL , Gagnon P .

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In eastern Canada, the destruction of kelp beds by dense aggregations (fronts) of the omnivorous green sea urchin, Strongylocentrotus droebachiensis, is a key determinant of the structure and dynamics of shallow reef communities. Recent studies suggest that hydrodynamic forces, but not sea temperature, determine the strength of urchin-kelp interactions, which deviates from the tenets of the metabolic theory of ecology (MTE). We tested the hypothesis that water temperature can predict short-term kelp bed destruction by S. droebachiensis in calm hydrodynamic environments. Specifically, we experimentally determined relationships among water temperature, body size, and individual feeding in the absence of waves, as well as among wave velocity, season, and aggregative feeding. We quantified variation in kelp-bed boundary dynamics, sea temperature, and wave height over three months at one subtidal site in Newfoundland to test the validity of thermal tipping ranges and regression equations derived from laboratory results. Consistent with the MTE, individual feeding during early summer (June-July) obeyed a non-linear, size- and temperature-dependent relationship: feeding in large urchins was consistently highest and positively correlated with temperature <12°C and dropped within and above the 12-15°C tipping range. This relationship was more apparent in large than small urchins. Observed and expected rates of kelp loss based on sea temperature and urchin density and size structure at the front were highly correlated and differed by one order of magnitude. The present study speaks to the importance of considering body size and natural variation in sea temperature in studies of urchin-kelp interactions. It provides the first compelling evidence that sea temperature, and not only hydrodynamic forces, can predict kelp bed destruction by urchin fronts in shallow reef communities. Studying urchin-seaweed-predator interactions within the conceptual foundations of the MTE holds high potential for improving capacity to predict and manage shifts in marine food web structure and productivity.

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Genes referenced: LOC100887844 LOC100893907 LOC115925415 LOC585084

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	Fig 1. Oscillatory wave tank used in Experiment 2.(A) Position of the experimental area [3×4 grid of concrete tiles of 0.3×0.3×0.05 m each], and (B) relative positions of the kelp (Alaria esculenta) line, zone of maximum canopy cover [Canopy zone], and zone to which green sea urchins (Strongylocentrotus droebachiensis) were introduced prior to the onset of trials [Start zone]. The sequence at the bottom shows urchins at (C) t = 0 [prior to introducing the kelp line], (D) t = 1 h, and (E) t = 6 h [end] of a trial at a wave velocity of 0.1 m s-1 (see Materials and methods for details).
	Fig 2. Mean (+SE) feeding rate of small (25–35 mm t.d.) and large (45–60 mm t.d.) green sea urchins (Strongylocentrotus droebachiensis) on kelp (Alaria esculenta) in seawater at 3, 6, 9, 12, 15, and 18°C (Experiment 1).


	Fig 5. Change in the position of the kelp-barrens interface at Cape Boone Cove from 3 July to 25 September, 2012.Values directly below sampling dates are the mean distance (±SE) of the kelp-barrens interface relative to benchmark eyebolts in the urchin barrens (0 m). The depth across the grid (from 10 to 0 m along the y-axis) is from 4 to 9 m. Values in parentheses are the approximate depth (in m) of the kelp-barrens interface. Horizontal dashed lines indicate the mean distance of the kelp-barrens interface on the first sampling event (3 July).
	Fig 6. Change in mean daily sea temperature and significant wave height (SWH) at Cape Boone Cove from 1 July to 30 September, 2012.Sea temperature and wave height data were acquired every 30 and 2 minutes, respectively, with one temperature logger and one water level logger secured to the seabed at a depth of 9 m. The arrow indicates the date (11 September) that the tail end of Hurricane Leslie reached the southeastern tip of Newfoundland (note the sharp decline in sea temperature and slight increase in SWH associated with this event).
	Fig 7. Relationship between the density of green sea urchins (Strongylocentrotus droebachiensis) and sea temperature in each of the four zones sampled at Cape Boone Cove from 3 July to 13 September, 2012.Barrens: 0.2 m from benchmark eyebolts in the urchin barrens; Pre-front: 2 m from the lower edge of the kelp bed; Front: at the leading edge of the urchin front; and Bed: 2 m into the kelp bed (see Table 6 for details of the regression analyses).
	Fig 8. Relationship between observed and expected daily rates of kelp loss during the summer 2012 survey at Cape Boone Cove (CBC) with and without data from 25 September, the last sampling event, which was 14 days after the passage of the tail end of Hurricane Leslie.Observed rates were calculated from our observational dataset at CBC, whereas expected rates were calculated with the equations derived from Experiment 1 (see Statistical analysis and Table 1 for details of the observational dataset and equations used and Table 7 for details of the two regression lines shown).
	Fig 9. Observed and expected (+SE) daily rates of kelp loss and mean (±SE) sea temperature for each of the six sampling intervals during the summer 2012 survey at Cape Boone Cove.Sampling intervals 5 (29 Aug—13 Sep) and 6 (13 Sep—25 Sep) include data acquired two and 14 days (on 13 and 25 September) after the passage of the tail end of Hurricane Leslie.

References [+] :

Blain, Canopy-forming seaweeds in urchin-dominated systems in eastern Canada: structuring forces or simple prey for keystone grazers? 2014, Pubmed, Echinobase