Response of Fish and Invertebrate Larvae to Backreef Sounds at Varying Distances: Implications for Habitat Restoration

Underwater sound is used by many marine larvae to orient to coastal habitats including backreef, sponge-dominated hardbottom habitat in the Florida Keys (FL, United States)—a particularly “noisy” coastal habitat. However, the distance over which acoustic cues are attractive to settlement-stage larvae is generally unknown. We examined this phenomenon in a region of the Florida Keys where mass sponge die-offs have diminished both underwater soundscapes and larval settlement. The absence of pronounced hardbottom-associated sound over such a large area allowed us to experimentally test in situ the response of fish and invertebrate larvae to broadcasted sounds at different distances from their source. We first measured the signal-to-noise ratio of healthy hardbottom habitat soundscapes broadcast from an underwater speaker at seven distances to determine the maximum range of the signal. Based on those results, larval collectors were then deployed at 10, 100, 500, and 1,000 m from speakers broadcasting sounds recorded at either degraded or healthy hardbottom sites for five consecutive nights during each of three new and full moon periods in summer/fall 2019. Larval settlement onto those collectors was affected by lunar phase and soundscape type, but varied among species. In most cases, the effect was small and not likely to be ecologically significant. The absence of a strong larval settlement response to a sound cue lies in contrast to results from other studies. We suspect that the small (\u3c500 m) radius of the broadcasted soundscapes may have limited the magnitude of the larval response to locally available larvae whose abundance may have been low because the experiment was conducted within a large, relatively quiet seascape. If true, it is possible that planktonic larvae may require a series of acoustic “sign-posts,” perhaps in combination with other cues (e.g., chemical), to successfully orient to distant nursery habitats. Although habitat restoration efforts may be able to restore healthy soundscapes, the typically small size and number of restoration sites may limit the range of the acoustic cue and thus larval attraction to restored habitats

Habitat Restoration Restores Underwater Soundscapes and Larval Recruitment

Habitat degradation alters many ecosystem processes, and the potential for the reestablishment of ecosystem function through restoration is an area of active research. Among marine systems, coastal habitats are particularly vulnerable to anthropogenic degradation and, in response, are the focus of marine ecological restoration. One of the crucial functions of structurally complex coastal habitats (e.g., saltmarshes, seagrass meadows, kelp forests, coral reefs) are as nurseries to coastal and offshore species, many of whose larvae utilize sound to locate suitable nursery habitat. However, the effect of habitat degradation and subsequent restoration on underwater soundscapes and their function as navigational cues for larvae is unexplored. We investigated these phenomena in sponge-dominated hardbottom habitat in the waters surrounding the middle Florida Keys (Florida, United States) that have been degraded in recent decades by massive sponge die-offs caused by harmful algal blooms. One of the consequences of sponge die-offs are dramatic changes in underwater sounds normally produced by sponge-associated animals. We tested whether soundscapes from healthy hardbottom habitat influenced larval recruitment, and then examined how hardbottom degradation and restoration with transplanted sponges affected underwater soundscapes and the recruitment of larval fishes and invertebrates. Larval assemblages recruiting to healthy areas were significantly different than those assemblages recruiting to either degraded or restored hardbottom areas. Fewer larvae recruited to degraded and restored areas compared to healthy hardbottom, particularly during the full moon. Experimental playback of healthy hardbottom soundscapes on degraded sites did not promote larval community differences although some individual species responded to the playback of healthy habitat soundscapes. These results indicate that habitat-associated soundscapes have idiosyncratic effects on larval settlement, which is diminished by the degradation of nursery habitat but can be reestablished with appropriate habitat restoration

A Planetary Companion to the Nearby M4 Dwarf, Gliese 876

Doppler measurements of the M4 dwarf star, Gliese 876, taken at both Lick and Keck Observatory reveal periodic, Keplerian velocity variations with a period of 61 days. The orbital fit implies that the companion has a mass of, M = 2.1 MJUP /sin i, an orbital eccentricity of, e = 0.27+-0.03, and a semimajor axis of, a = 0.21 AU. The planet is the first found around an M dwarf, and was drawn from a survey of 24 such stars at Lick Observatory. It is the closest extrasolar planet yet found, providing opportunities for follow--up detection. The presence of a giant planet on a non-circular orbit, 0.2 AU from a 1/3 M_Sun star, presents a challenge to planet formation theory. This planet detection around an M dwarf suggests that giant planets are numerous in the Galaxy.Comment: 13 pages, 3 Figure

Belowground Growth Strategies of Native and Invasive Rhizomatous perennial grasses in response to precipitation variability, clipping, and competition

Invasive clonal species may exhibit different growth strategies than their native clonal competitors. In this study, we examined the spatial distribution of tiller outgrowth and the bud bank by comparing the investment in phalanx versus guerilla growth of a native and invasive perennial grass in North America. We also examined the efect of altered precipitation frequency, clipping, and competition on their clonal growth strategies. Investment in phalanx and guerilla growth was assessed by examining live propagule and tiller production from the plant crown versus its rhizomes. Although invasive Bromus inermis and native Pascopyrum smithii exhibited similar clonal growth strategies as young seedlings, their clonal growth strategies signifcantly difered by the end of their frst growing season. Pascopyrum smithii invested in dual phalanx and guerilla tiller outgrowth and bud placement, and B. inermis primarily invested in phalanx tiller outgrowth and bud placement. Competition rather than intra-annual precipitation variability and clipping altered the clonal growth strategy of these species. Intra- and inter- specifc competition did not alter tiller outgrowth for either species. However, inter-specifc competition caused both species to alter their bud placement. Bromus inermis shifted more buds from phalanx to guerilla positions while P. smithii shifted in the opposite direction. This may enable invasive B. inermis to expand while confning native P. smithii to more localized areas in the future. Clonal growth strategies appear to be species specifc and responsive to inter-specifc competition. Investigating the belowground bud aspect of clonal growth can reveal the mechanism driving the future aboveground clonal growth strategy of native and invasive rhizomatous grasses and help inform the patterns of invasion within a plant community

Belowground Mechanism Reveals Climate Change Impacts on Invasive Clonal Plant Establishment

Climate change and disturbance can alter invasion success of clonal plants by differentially affecting the clonal traits influencing their establishment as young plants. Clonal traits related to the vegetative reproduction of native Pascopyrum smithii and non-native Bromus inermis grass seedlings were evaluated under altered precipitation frequencies and a single grazing event. Pascopyrum smithii maintained similar vegetative reproduction under three simulated precipitation frequencies whereas B. inermis vegetative reproduction declined as precipitation became more intermittent. Vegetative reproduction of the non-native B. inermis was greater than the native P. smithii under all simulated precipitation frequencies except the most intermittent scenario. A single grazing event did not affect either species’ response to intra-annual precipitation variability but did slightly reduce their clonal growth and increase their bud dormancy. In young plants, clonal traits of the invasive grass favored its superior expansion and population growth compared to the native grass except under the most severe climate change scenario. Grassland restoration using native P. smithii seeds would be successful in most years due to its resilient clonal growth in a changing climate. Clonal infrastructure development in young plants is critical to clonal plant establishment and persistence in a changing climate and under disturbed conditions