“Internal tide pools” prolong kelp forest hypoxic events

Abstract

This study uses field observations within a single kelp bed in southern Monterey Bay, California, to evaluate the retention of cold, hypoxic water within depressions in the rocky reef following relaxation of internal wave events. Just as tide pools in the rocky intertidal zone persist in depressions following the relaxation of surface waves and tides, "internal tide pools" persist in depressions in the subtidal reef following the relaxation of internal waves. When internal waves contain low dissolved oxygen (DO) water, the duration of hypoxic events can be extended for organisms within the pooling zone. Additionally, we suggest that internal wave run-up can cause the early arrival of cold hypoxic water in these same areas, thereby extending hypoxic events further. Together, internal tide pools and wave run-up extend event durations by 20% on average. However, some events are extended up to 160% of the regional-scale event length, which may be more ecologically relevant than the mean extension. When internal waves contain other environmental covariates (e.g., low pH, nutrients), internal tide pools are expected to create a similar time extension and patchy landscape in these quantities. We attribute the slowing of the relaxation of sub-thermocline water within pooling zones to retention within the reef itself rather than a drag effect of the kelp bed. Following hypoxic internal bores, internal tide pools cause a period of extreme spatial variability in the DO and temperature field which is likely to impact the distribution and behavior of local organisms.

Short- and long-term impacts of variable hypoxia exposures on kelp forest sea urchins

“…However, sea urchins that spent longer continuous periods of time in ambient oxygen conditions produced larger gonads, suggesting that sea urchin reproductive processes may be sensitive to higher-frequency patterns of variability in sublethal hypoxia. Therefore, present-day spatial differences in the temporal patterns of hypoxia exposure 19,21,22,52 and future projected changes in DO exposure regimes [27][28][29] could produce population-level effects (through reproduction) and ecosystem-level effects (e.g., trophic energy transfer) through impacts on just one or two particularly sensitive organism-level responses, such as gonad production. More generally, these results show that different processes may be resilient to hypoxia, or, on the contrary, close to a tipping point of rapid change.…”

“…Conversely, some regimes of fluctuating exposures to low pH can also be more energetically stressful than constant exposures to the same conditions 17 . Global change is altering the natural variability in environmental conditions across ecosystems 2 , and local-scale processes can further mediate these changes to produce small-scale variation in the temporal patterns of organisms' exposure to physiological stress, potentially creating local stressor hotspots or refuges 2,[18][19][20][21][22] . Therefore, it is important to understand how different patterns of variability in climatic stressors impact organisms and their roles in ecosystems.…”

“…Lethal exposures have led to a few mass mortalities in some nearshore California Current ecosystems 6,25,30 , but sublethal exposures and impacts are likely to be much more common 8 . Although large-scale upwelling processes are responsible for advecting deep, offshore hypoxic water towards the surface, smaller-scale nearshore physical and biological processes interact with local bathymetry to generate highly variable patterns of exposure to hypoxia in coastal habitats 19,26,31 , which can differ significantly even at small spatial scales 19,21,22 . However, the consequences of these sublethal and temporally variable hypoxia exposures for nearshore California Current species and ecosystems are not well known.…”

High‐Resolution Observations of Subsurface Fronts and Alongshore Bottom Temperature Variability Over the Inner Shelf

“…Internal bores contribute to cross‐shore heat fluxes (Nam & Send, ), sediment resuspension (Bogucki et al, ), and dissolved oxygen variability (Walter et al, ). In the shallow kelp forest environment, the effects of irregular, rocky substrate on internal bores lead to small‐scale physical and biogeochemical heterogeneity (Leary et al, ). However, most studies have focused on evolution of fronts in the cross‐shelf direction, in which gradients tend to be strongest.…”

An Enhanced Ocean Acidification Observing Network: From People to Technology to Data Synthesis and Information Exchange

“…In addition, and particularly regarding marine benthic organisms, seawater physics and chemistry may significantly vary across small microclimates within habitats. Deployment of arrays of multiple sensors may help characterize these systems (e.g., Leary et al, 2017). Combining sensor data with biology remains an important but very young area of research, and often requires interdisciplinary collaborations or advanced training.…”