Warm microhabitats drive both increased respiration and growth rates of intertidal consumers

Abstract

Rocky intertidal organisms are often exposed to broadly fluctuating temperatures as the tides rise and fall. Many mobile consumers living on the shore are immobile during low tide, and can be exposed to high temperatures on calm, warm days. Rising body temperatures can raise metabolic rates, induce stress responses, and potentially affect growth and survival, but the effects may differ among species with different microhabitat preferences. We measured aerial and aquatic respiration rates of 4 species of Lottia limpets from central California, and estimated critical thermal maxima. In a variety of microhabitats in the field, we tracked body temperatures and measured limpet growth rates on experimental plates colonized by natural microalgae. Limpet species found higher on the shore had lower peak respiration rates during high temperature aerial exposure, and had higher critical thermal maxima. Using our long-term records of field body temperatures, we estimated cumulative respiration to be 5 to 14% higher in warm microhabitats. Growth rates in the field appear to be driven by an interaction between available microalgal food resources, low tide temperature, and limpet species identity, with limpets from warmer microhabitats responding positively to higher food availability and higher low tide temperatures. Stressful conditions in warm microhabitats make up a small portion of the total lifetime of these limpets, but the greater proportion of time spent at non-stressful, but warm, body temperatures may result in enhanced growth compared to limpets living in cooler microhabitats.

Quantifying the top-down effects of grazers on a rocky shore: selective grazing and the potential for competition

“…However, limpet grazing reduced F 0 values to those typical of warm, ungrazed plates, regardless of temperature, suggesting that in intertidal regions where grazers are very sparse, F 0 values will be low if the region is warm, and high if the region is cool. The presence of grazers masks this signal, corroborating prior observations that limpet grazing suppresses natural variation in the density of the MPB along intertidal shores (Miller et al 2015). Our results also indicate that a low F 0 value on a warm, ungrazed plate does not necessarily signify a change in assemblage structure at the microscopic level.…”

“…The L. austrodigitalis-grazed plates suitable for C:N analysis were initially seeded with an average of 8.4 limpets while the L. scabra-grazed plates were seeded only with 4.4 limpets, but by the end of our study, the number of limpets on L. austrodigitalis and L. scabra-grazed plates were statistically indistinguishable (3.53 ± 0.35). This was likely due to the fact that L. austrodigitalis have a higher propensity for escaping our field enclosures than L. scabra (Miller et al 2015). Additionally, no relationship was found between the number of limpets remaining on the plates at the end of the study and C:N ratio.…”

“…MPB is reduced in biomass and therefore less fluorescent when exposed to warm conditions (Jackson et al 2010, Miller et al 2015. However, limpet grazing reduced F 0 values to those typical of warm, ungrazed plates, regardless of temperature, suggesting that in intertidal regions where grazers are very sparse, F 0 values will be low if the region is warm, and high if the region is cool.…”

“…To minimize the possibility of interactions arising from spatial differences in temperature, we eliminated site 6, which was significantly warmer than the other sites, from our ANOVA. However, temperature and dark-adapted fluorescence are known to be negatively correlated (Miller et al 2015), so we ran an additional analysis of covariance to determine whether the relationship between temperature and dark-adapted fluorescence was likely influenced by grazing treatment. The data from site 6 were included in this test to provide a larger temperature range.…”

“…Dark-adapted fluorescence of intertidal MPB can be used as a proxy for chlorophyll a density and is known to decrease as temperature increases (Miller et al 2015). This suggests that less food is available to epilithic grazers like limpets when rock surface temperatures are high.…”