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Ocean acidification, a reduction in the pH of the oceans caused by increasing CO2, can have negative physiological effects on marine species. In this study, we examined how CO2-driven acidification affected the growth and survival of juvenile golden king crab (Lithodes aequispinus), an important fishery species in Alaska. Juveniles were reared from larvae in surface ambient pH seawater at the Kodiak Laboratory. Newly molted early benthic instar crabs were randomly assigned to one of three pH treatments: (1) surface ambient pH ~ 8.2, (2) likely in situ ambient pH 7.8, and (3) pH 7.5. Thirty crabs were held in individual cells in each treatment for 127 days and checked daily for molting or death. Molts and dead crabs were photographed under a microscope and measured using image analysis to assess growth and morphology. Mortality was primarily associated with molting in all treatments, differed among all treatments, and was highest at pH 7.5 and lowest at ambient pH. Crabs at pH 7.5 were smaller than crabs at ambient pH at the end of the experiment, both in terms of carapace length and wet mass; had a smaller growth increment after molting; had a longer intermolt period. Carapace morphology was not affected by pH treatment. Decreased growth and increased mortality in laboratory experiments suggest that lower pH could affect golden king crab stocks and fisheries. Future work should examine if larval rearing conditions affect the juvenile response to low pH.

Oceanic and coastal waters are acidifying due to processes dominated in the open ocean by increasing atmospheric CO₂ and dominated in estuaries and some coastal waters by nutrient-fueled respiration. The patterns and severity of acidification, as well as its effects, are modified by the host of stressors related to human activities that also influence these habitats. Temperature, deoxygenation, and changes in food webs are particularly important co-stressors because they are pervasive, and both their causes and effects are often mechanistically linked to acidification. Development of a theoretical underpinning to multiple stressor research that considers physiological, ecological, and evolutionary perspectives is needed because testing all combinations of stressors and stressor intensities experimentally is impossible. Nevertheless, use of a wide variety of research approaches is a logical and promising strategy for improving understanding of acidification and its effects. Future research that focuses on spatial and temporal patterns of stressor interactions and on identifying mechanisms by which multiple stressors affect individuals, populations, and ecosystems is critical. It is also necessary to incorporate consideration of multiple stressors into management, mitigation, and adaptation to acidification and to increase public and policy recognition of the importance of addressing acidification in the context of the suite of other stressors with which it potentially interacts.

We used flow cytometry to determine if there would be a difference in hematology, selected immune functions, and hemocyte pH (pHi), under two different, future ocean acidification scenarios (pH = 7.50, 7.80) compared to current conditions (pH = 8.09) for Chionoecetes bairdi, Tanner crab. Hemocytes were analyzed after adult Tanner crabs were held for two years under continuous exposure to acidified ocean water. Total counts of hemocytes did not vary among control and experimental treatments; however, there were significantly greater number of dead, circulating hemocytes in crabs held at the lowest pH treatment. Phagocytosis of fluorescent microbeads by hemocytes was greatest at the lowest pH treatment. These results suggest that hemocytes were dying, likely by apoptosis, at a rate faster than upregulated phagocytosis was able to remove moribund cells from circulation at the lowest pH. Crab hemolymph pH (pHe) averaged 8.09 and did not vary among pH treatments. There was no significant difference in internal pH (pHi) within hyalinocytes among pH treatments and the mean pHi (7.26) was lower than the mean pHe. In contrast, there were significant differences among treatments in pHi of the semi-granular+granular cells. Control crabs had the highest mean semi-granular+granular pHi compared to the lowest pH treatment. As physiological hemocyte functions changed from ambient conditions, interactions with the number of eggs in the second clutch, percentage of viable eggs, and calcium concentration in the adult crab shell was observed. This suggested that the energetic costs of responding to ocean acidification and maintaining defense mechanisms in Tanner crab may divert energy from other physiological processes, such as reproduction.

Blue king crabs (Paralithodes platypus) are commercially and ecologically important in Alaska, USA, but population abundances have fluctuated over the past several decades likely resulting from a combination of environmental and biological factors, including recruitment variability. Cannibalism between cohorts may be a source of mortality limiting recruitment success in the wild, but the degree of inter-cohort cannibalism is unknown for early benthic phase blue king crabs. In laboratory experiments, we evaluated the effects of habitat type (sand and shell) on the predator functional response and foraging behavior of year-1 blue king crabs as predators of year-0 conspecifics and examined the effects of predator presence on crypsis of prey crabs. In sand, consumption rates increased with predator size and prey density until satiation, while predation rates in shell were low regardless of predator size or prey density. These differential predation rates yielded a type III functional response in sand but a type I functional response in shell habitat. Crypsis of prey crabs was generally high and did not change in the presence of predators. Predator foraging activity was reduced in shell and may be an adaptive behavior to balance foraging efficiency and susceptibility to larger predators. Our results demonstrate that early benthic phase blue king crabs are cannibalistic between cohorts in the laboratory and that shell material is extremely effective for reducing encounter rates with conspecific predators. The distribution and abundance of such habitat may be important for recruitment success in some populations. Future studies should compare benthic habitat and species assemblages in areas with variable abundances, such as the Pribilof Islands and Saint Matthew Island in the eastern Bering Sea, to better understand possible mechanisms for recruitment variability.

Invasive weeds threaten the biodiversity and forage productivity of grasslands worldwide. However, management of these weeds is constrained by the practical difficulty of detecting small-scale infestations across large landscapes and by limits in understanding of landscape-scale invasion dynamics, including mechanisms that enable patches to expand, contract, or remain stable. While high-end hyperspectral remote sensing systems can effectively map vegetation cover, these systems are currently too costly and limited in availability for most land managers. We demonstrate application of a more accessible and cost-effective remote sensing approach, based on simple aerial imagery, for quantifying weed cover dynamics over time. In California annual grasslands, the target communities of interest include invasive weedy grasses (Aegilops triuncialis and Elymus caput-medusae) and desirable forage grass species (primarily Avena spp. and Bromus spp.). Detecting invasion of annual grasses into an annual-dominated community is particularly challenging, but we were able to consistently characterize these two communities based on their phenological differences in peak growth and senescence using maximum likelihood supervised classification of imagery acquired twice per year (in mid- and end-of season). This approach permitted us to map weed-dominated cover at a 1-m scale (correctly detecting 93% of weed patches across the landscape) and to evaluate weed cover change over time. We found that weed cover was more pervasive and persistent in management units that had no significant grazing for several years than in those that were grazed, whereas forage cover was more abundant and stable in the grazed units. This application demonstrates the power of this method for assessing fine-scale vegetation transitions across heterogeneous landscapes. It thus provides means for small-scale early detection of invasive species and for testing fundamental questions about landscape dynamics.

Assessing decapod sensitivity to regional-scale ocean acidification (OA) conditions is limited because of a fragmented understanding of the thresholds at which they exhibit biological response. To address this need, we undertook a three-step data synthesis: first, we compiled a dataset composed of 27,000 datapoints from 55 studies of decapod responses to OA. Second, we used statistical threshold analyses to identify OA thresholds using pH as a proxy for 13 response pathways from physiology to behavior, growth, development and survival. Third, we worked with the panel of experts to review these thresholds, considering the contributing datasets based on quality of the study, and assign a final thresholds and associated confidence scores based on quality and consistency of findings among studies. The duration-dependent thresholds were within a pH range from 7.40 to 7.80, ranging from behavioral and physiological responses to mortality, with many of the thresholds being assigned medium-to-high confidence. Organism sensitivity increased with the duration of exposure but was not linked to a specific life-stage. The thresholds that emerge from our analyses provide the foundation for consistent interpretation of OA monitoring data or numerical ocean model simulations to support climate change marine vulnerability assessments and evaluation of ocean management strategies.

Red and blue king crab ( Paralithodes camtschaticus and P. platypus ) are both important fisheries species in Alaska. Despite the threat of climate change, the upper thermal tolerance of these species has been little studied. In this paper, we determine the lethal and sub-lethal thermal responses of year-0 juvenile red and blue king crab. Crab were exposed to a range of temperatures for 24 h to determine the lethal temperature at which mortality was 50% (LT 50 ). Feeding rations were determined at sub-lethal temperatures for both species. Finally, crab were held for 45 days at sub-lethal temperatures and growth and long-term mortality rates were measured. The LT 50 s of red and blue king crab were 24.3 and 21.3 °C, respectively. Feeding ration peaked between 12 and 17 °C for red king crab and between 5 and 10 °C for blue king crab. The growth rate of red king crab decreased at temperatures above 12 °C, while blue king crab exhibited very low growth rates at temperatures 12 °C and above. Increased mortality in the 45-day experiment occurred for red king crab at 20 °C and blue king crab at 17 °C. This study suggests that, within their current distribution, the juveniles of both species could withstand warming of several degrees and not be thermally stressed; however, more research into the effect of increased temperature variability and other response variables, such as resting and active respiration, is needed. In addition, increased temperature could affect other life-history stages differently or have indirect effects, such as altered species interactions and food availability.

Structural and mechanical properties of the decapod exoskeleton affect foraging, defense, and locomotion. Ocean acidification (OA) poses a threat to marine biomes and their inhabitants, particularly calcifying organisms. Vulnerability of the snow crab, Chionecetes opilio, a commercially important, high-latitude species, to OA has not been explored. Although all oceans are experiencing acidification, abiotic factors in high-latitude areas increase the rate of acidification. We examined the effect of long-term (2 year) exposure to decreased seawater pH (7.8 and 7.5, PCO2 ~ 760 and 1550 µatm, respectively) on exoskeletal properties in post-terminal-molt female C. opilio. Since the effects of OA vary among body regions in decapods, exoskeletal properties (microhardness, thickness, and elemental composition) were measured in five body regions: the carapace, both claws, and both third walking legs. Overall, adult C. opilio exoskeletons were robust to OA in all body regions. Decreased pH had no effect on microhardness or thickness of the exoskeleton, despite a slight (~ 6%) reduction in calcium content in crabs held at pH 7.5. In contrast, exoskeletal properties varied dramatically among body regions regardless of pH. The exoskeleton of the claws was harder, thicker, and contained more calcium but less magnesium than that of other body regions. Exoskeleton of the legs was thinner than that of other body regions and contained significantly greater magnesium concentrations (~ 2.5 times higher than the claws). Maintenance of exoskeletal properties after long-term OA exposure, at least down to pH 7.5, in adult C. opilio suggests that wild populations may tolerate future ocean pH conditions.

Hypoxia is an environmental stressor that affects abundance, biomass, diversity, and ecosystem function of benthic assemblages worldwide, yet its collective impact at individual, population, and ecosystem levels has rarely been investigated. We examined the effects of hypoxia on the biomass-dominant clam, Macoma balthica , in the York and Rappahannock Rivers (Chesapeake Bay, USA). We (1) surveyed the M. balthica populations in both rivers in 2003 and 2004, (2) determined the effects of low dissolved oxygen (DO) on M. balthica fecundity in a laboratory experiment, and (3) employed a predator-exclusion field experiment to establish the effects of hypoxia and prey density on predation upon M. balthica . The resultant data were used to parameterize a matrix model, which was analyzed to define potential effects of hypoxia at the population level. In both rivers, hypoxia decreased individual clam growth and caused local extinction of populations. Hypoxia reduced egg production of M. balthica by 40% and increased protein investment per egg. In the predator-exclusion field experiment, hypoxia magnified predation rates threefold and altered the functional response of predators to M. balthica from a stabilizing type III functional response to a destabilizing type II functional response. In a density-independent matrix model, hypoxia resulted in coupled source-sink metapopulation dynamics, with hypoxic areas acting as black-hole sinks. Increases in the spatial and temporal extent of hypoxia caused the populations to decline toward extinction. In a second model that incorporated density dependence, under mild hypoxic conditions trophic transfer from M. balthica to predators increased, but at increased spatial or temporal extent of hypoxia trophic transfer decreased. The major decline in trophic transfer to predators under severe hypoxia resulted from diversion of M. balthica biomass into the microbial loop. Our model predicted that there are multiple stable states for M. balthica populations (high and very low densities), such that the saddle point (threshold at which the population switches from one state to the other) increased and resilience decreased with the spatial extent of hypoxia. We underscore how effects of a stressor at the individual level can combine to have substantial population and ecosystem-level effects.