The mussel Mytilus californianus is a competitive dominant on wave—swept rocky intertidal shores. Mussel beds may exist as extensive monocultures; more often they are an everchanging mosaic of many species which inhabit wave—generated patches or gaps. This paper describes observations and experiments designed to measure the critical parameters of a model of patch birth and death, and to use the model to predict the spatial structure of mussel beds. Most measurements were made at Tatoosh Island, Washington, USA, from 1970—1979. Patch size ranged at birth from a single mussel to 38 m2; the distribution of patch sizes approximates the lognormal. Birth rates varied seasonally and regionally. At Tatoosh the rate of patch formation varied during six winters from 0.4—5.4% of the mussels removed per month. The disturbance regime during the summer and at two mainland sites was 5—10 times less. Annual disturbance patterns tended to be synchronous within 11 sites on one face of Tatoosh over a 10—yr interval, and over larger distances (16 km) along the coastline. The pattern was asynchronous, however, among four Tatoosh localities. Patch birth rate, and mean and maximum size at birth can be used as adequate indices of disturbance. Patch disappearance (death) occurs by three mechanisms. Very small patches disappear almost immediately due to a leaning response of the border mussels (0.2 cm/d). Intermediate—sized patches (<3.0 m2) are eventually obliterated by lateral movement of the peripheral mussels: estimates based on 94 experimental patches yield a mean shrinking rate of 0.05 cm/d from each of two principal dimensions. Depth of the adjacent mussel bed accounts for much of the local variation in closing rate. In very large patches, mussels must recruit as larvae from the plankton. Recovery begins at an average patch age of 26 mo; rate of space occupation, primarily due to individual growth, is 2.0—2.5%/mo. Winter birth rates suggest a mean turnover time (rotation period) for mussel beds varying from 8.1—34.7 yr, depending on the location. The minimal value is in close agreement with both observed and calculated minimal recovery times. Projections of total patch area, based on the model, are accurate to within 5% of the observed. Using a method for determining the age of patches, based on a growth curve of the barnacle Balanus cariosus, the model permits predictions of the age—size structure of the patch population. The model predicts with excellent resolution the distribution of patch area in relation to time since last disturbance. The most detailed models which include size structure within age categories are inconclusive due to small sample size. Predictions are food for large patches, the major determinants of environmental patterns, but cannot deal adequately with smaller patches because of stochastic effects. Colonization data are given in relation to patch age, size and intertidal position. We suggest that the reproductive season of certain long—lived, patch—dependent species is moulded by the disturbance regime. The necessary and vital connection between disturbance which generates spatial pattern and species richness in communities open to invasion is discussed.

Intertidal landscapes: disturbance and the dynamics of pattern [Mytilus californianus, mussels]

Mass budget calculations, validated with satellite gravity observations (from the Gravity Recovery and Climate Experiment (GRACE) satellites), enable us to quantify the individual components of recent Greenland mass loss. The total 2000-2008 mass loss of ~1500 gigatons, equivalent to 0.46 millimeters per year of global sea level rise, is equally split between surface processes (runoff and precipitation) and ice dynamics. Without the moderating effects of increased snowfall and refreezing, post-1996 Greenland ice sheet mass losses would have been 100% higher. Since 2006, high summer melt rates have increased Greenland ice sheet mass loss to 273 gigatons per year (0.75 millimeters per year of equivalent sea level rise). The seasonal cycle in surface mass balance fully accounts for detrended GRACE mass variations, confirming insignificant subannual variation in ice sheet discharge.

/pdf/partitioning-recent-greenland-mass-loss-2yz2ywoqgy.pdf

Partitioning Recent Greenland Mass Loss

Recent observations in Greenland have led to the notion that surface meltwater supply to the ice-sheet bed lubricates ice flow, suggesting that climate warming could lead to runaway glacial acceleration. Christian Schoof uses a physically based model that captures drainage channelization under the ice to challenge this view. The model shows that increased melt supply leads to channelization and a drop in water pressure. This causes ice flow to slow down rather than speed up. However, this effect can be overcome if melt supply is variable over short timescales, when temporary pressure spikes can lead to accelerated flow. The positive melt/dynamic thinning feedback is therefore still viable, but is heavily dependent on daily temperature variations and rain events that are largely ignored in current ice sheet models. Increased melting is often assumed to cause acceleration of ice sheets and glaciers through basal lubrication, possibly leading to increased rates of sea level rise. Now a physically-based model challenges this view, illustrating that above a critical threshold, increased melt will suppress the dynamic thinning process. Short-term spikes in water delivery, as from lake drainage or precipitation, still have the potential to generate spikes in velocity, but overall increases in melt do not appear likely to cause velocity increases. Increased ice velocities in Greenland1 are contributing significantly to eustatic sea level rise. Faster ice flow has been associated with ice–ocean interactions in water-terminating outlet glaciers2 and with increased surface meltwater supply to the ice-sheet bed inland. Observed correlations between surface melt and ice acceleration2,3,4,5,6 have raised the possibility of a positive feedback in which surface melting and accelerated dynamic thinning reinforce one another7, suggesting that overall warming could lead to accelerated mass loss. Here I show that it is not simply mean surface melt4 but an increase in water input variability8 that drives faster ice flow. Glacier sliding responds to melt indirectly through changes in basal water pressure9,10,11, with observations showing that water under glaciers drains through channels at low pressure or through interconnected cavities at high pressure12,13,14,15. Using a model that captures the dynamic switching12 between channel and cavity drainage modes, I show that channelization and glacier deceleration rather than acceleration occur above a critical rate of water flow. Higher rates of steady water supply can therefore suppress rather than enhance dynamic thinning16, indicating that the melt/dynamic thinning feedback is not universally operational. Short-term increases in water input are, however, accommodated by the drainage system through temporary spikes in water pressure. It is these spikes that lead to ice acceleration, which is therefore driven by strong diurnal melt cycles4,14 and an increase in rain and surface lake drainage events8,17,18 rather than an increase in mean melt supply3,4.

/pdf/ice-sheet-acceleration-driven-by-melt-supply-variability-40af4rihmc.pdf

Ice-sheet acceleration driven by melt supply variability

Using RADARSAT synthetic aperture radar data, we have mapped the flow velocity over much of the Greenland ice sheet for the winters of 2000/01 and 2005/06. These maps provide a detailed view of the ice-sheet flow, including that of the hundreds of glaciers draining the interior. The focused patterns of flow at the coast suggest a strong influence of bedrock topography. Differences between our two maps confirm numerous early observations of accelerated outlet glacier flow as well as revealing previously unrecognized changes. The overall pattern is one of speed-up accompanied by terminus retreat, but there are also several instances of surge behavior and a few cases of glacier slowdown. Comprehensive mappings such as these, at regular intervals, provide an important new observational capability for understanding ice-sheet variability.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/B364931057A783211C28147B37A88CA3/S0022143000207065a.pdf/div-class-title-greenland-flow-variability-from-ice-sheet-wide-velocity-mapping-div.pdf

Greenland flow variability from ice-sheet-wide velocity mapping

[1] High-resolution (∼11 km) regional climate modeling shows total annual precipitation on the Greenland ice sheet for 1958–2007 to be up to 24% and surface mass balance up to 63% higher than previously thought. The largest differences occur in coastal southeast Greenland, where the much higher resolution facilitates capturing snow accumulation peaks that past five-fold coarser resolution regional climate models missed. The surface mass balance trend over the full 1958–2007 period reveals the classic pattern expected in a warming climate, with increased snowfall in the interior and enhanced runoff from the marginal ablation zone. In the period 1990–2007, total runoff increased significantly, 3% per year. The absolute increase in runoff is especially pronounced in the southeast, where several outlet glaciers have recently accelerated. This detailed knowledge of Greenland's surface mass balance provides the foundation for estimating and predicting the overall mass balance and freshwater discharge of the ice sheet.

/pdf/higher-surface-mass-balance-of-the-greenland-ice-sheet-bjht8weoxh.pdf

Higher surface mass balance of the Greenland ice sheet revealed by high-resolution climate modeling.

Continuous Global Positioning System observations reveal rapid and large ice velocity fluctuations in the western ablation zone of the Greenland Ice Sheet. Within days, ice velocity reacts to increased meltwater production and increases by a factor of 4. Such a response is much stronger and much faster than previously reported. Over a longer period of 17 years, annual ice velocities have decreased slightly, which suggests that the englacial hydraulic system adjusts constantly to the variable meltwater input, which results in a more or less constant ice flux over the years. The positive-feedback mechanism between melt rate and ice velocity appears to be a seasonal process that may have only a limited effect on the response of the ice sheet to climate warming over the next decades.

https://science.sciencemag.org/content/sci/321/5885/111.full.pdf

Large and Rapid Melt-Induced Velocity Changes in the Ablation Zone of the Greenland Ice Sheet

Coastal dune dynamics in response to excavated foredune notches

Wave forcing over an intertidal mussel bed

Measuring spatial and temporal variation in surface moisture on a coastal beach with a near-infrared terrestrial laser scanner

Coastal foredunes are highly dynamic landforms because of rapid erosion by waves and currents during storm surges in combination with gradual accretion by aeolian transport during more quiescent conditions. While our knowledge into the mechanisms behind foredune erosion have reached considerable maturity, this is not the case for foredune growth. High resolution spatio-temporal data sets of beach and foredune topography, which are needed to increase our understanding of mechanisms behind aeolian transport in coastal environments and to develop predictive dune-accretion models, are scarce. Here we aim to illustrate that repeated Mobile Laser Scanning (MLS) surveys provide an accurate and robust method to study detailed changes in dune volume on the timescales of months to years. An MLS system attached to an inertial navigation system with RTK-GPS (INS-GPS) was used to carry out 13 surveys along a 3.5-km Dutch beach over a 2.5-year period. The height observations were post-processed and averaged into 1 × 1 m Digital Elevation Models (DEMs). Comparison with airborne LiDAR and RTK-GPS data revealed that the obtained DEMs were accurate and robust up to a height of 15 m in the foredune above which dense vegetation hampers the MLS to see the sand surface. Estimates of dune volume change of the lower 13 m of the foredune have an uncertainty of about 0.25 m 3 /m. Time series of dune volume change show that at our study site the foredune accretes throughout the year at similar rates (10 m 3 /m/year), while marine erosion is obviously confined to storm surges. Foredune accretion and erosion vary spatially, which can, in part, be related to variations in beach width.

https://www.mdpi.com/2077-1312/6/4/126/pdf

J.J.A. Donker

Papers

Large and Rapid Melt-Induced Velocity Changes in the Ablation Zone of the Greenland Ice Sheet

Coastal dune dynamics in response to excavated foredune notches

Wave forcing over an intertidal mussel bed

Measuring spatial and temporal variation in surface moisture on a coastal beach with a near-infrared terrestrial laser scanner

Spatio-Temporal Variations in Foredune Dynamics Determined with Mobile Laser Scanning