The use of historical bathymetric charts in a GIS to assess morphological change in estuaries

Navigational charts are a valuable data source for the study of morphological change in estuaries, as they provide bathymetric information of many estuaries, and span a vast period of time. Charts are suitable for studying patterns of morphological change, such as shoal and channel migration. In addition, sequential bathymetric charts or bed surveys can be used to calculate sedimentation and erosion rates. However, a number of problems arise when using these data to identify morphological change. Sources of error and uncertainty are associated with surveying techniques used, density of depth sampling points, interpolation and averaging during compilation of the chart. Large systematic errors may stem from the non-uniformity or poorly defined levels to which the depths on the charts are reduced. Errors propagate in spatiotemporal operations using such charts. Analysis of historical bathymetric charts from the Ribble estuary (north-west England) illustrates the difficulties involved in assessing morphological change in a quantitative manner. The pattern of morphological development within the Ribble estuary over the last 150 years was clear, and a significant long-term net accretional trend was found. However, temporal variations in the rate of sedimentation on a decadal scale were generally not significant.     

North American Trends in Extreme Precipitation

An analysis of extreme precipitation events indicates that there has been a sizable increase in their frequency since the 1920s/1930s in the U.S. There has been no discernible trend in the frequency of the most extreme events in Canada, but the frequency of less extreme events has increased in some parts of Canada, notably in the Arctic. In the U.S., frequencies in the late 1800s/early 1900s were about as high as in the 1980s/1990s. This suggests that natural variability of the climate system could be the cause of the recent increase, although anthropogenic forcing due to increasing greenhouse gas concentrations cannot be discounted as another cause. It is likely that anthropogenic forcing will eventually cause global increases in extreme precipitation, primarily because of probable increases in atmospheric water vapor content and destabilization of the atmosphere. However, the location, timing, and magnitude of local and regional changes remain unknown because of uncertainties about future changes in the frequency/intensity of meteorological systems that cause extreme precipitation.     

Response of juvenile fish assemblages in tidal salt marsh creeks treated forPhragmites removal

We examined fish assemblages in tidal salt marsh creeks in Delaware Bay in order to evaluate their response to treatment forPhragmites removal following initial treatment in 1996. In Alloway Crrek, a tributary to Delaware Bay, reference creeks draining marsh of untreatedPhragmites or naturally occurringSpartina were compared with creeks in marshes treated forPhragmites removal. These reference and treated creeks occur in close proximity and share many characteristics including salinity, temperature, dissolved oxygen, and turbidity, although creeks inPhragmites sites differed slightly in bathymetry. We analyzed a time series of otter trawl collections (22 monthly sample periods from 1999 to 2001) for differences in juvenile fish assemblage among creeks with different vegetation history. Periodically, young-of-the-year (YOY) and age 1+ white perch (Morone americana), YOY spot (Leiostomus xanthurus), YOY Atlantic menhaden (Brevoortia tyrannus), and other species were relatively more abundant atPhragmites sites, but other dominant species were preiodically abundant at all sites. Among-treatment differences based on principal response curves analysis accounted for about 19% of the total species variation, but differences varied widely among sample periods and there is little or no indication of a trend over the 3-yr period. Larger collections were often associated with subtidal structure, which was more common atPhragmites sites and potentially represents a sampling artifact. Assemblages of creeks with differing vegetation history differ weakly but recognizably, suggesting slow or little response to treatment, at least based on otter trawl collections in subtidal marsh creeks.

     

Recurrent shoaling and channel dredging, Middle and Upper Mississippi River, USA

A 34-year record of dredging in a 484-km reach of the Middle and Upper Mississippi River documents the spatial and temporal patterns of bed aggradation in an intensively engineered river. Between 1964 and 1997, 183  million m³ was removed from the study reach, with 112.6 km of the channel undergoing dredging of some kind, 12.1 km requiring ≥5 dredgings during this period, 2.6 km requiring ≥10 dredgings, and one site requiring 29 dredgings in 34 years. Forty-three sites were identified where dredging volume was exceptionally high and/or were frequently repeated. These sites occur in five settings: (1) where flow is divided through chutes or side channels; (2) at the mouths of largely unregulated tributary streams; (3) at thalweg crossings in meander bends; (4) in long, straight reaches of the channel; and (5) near problematic engineered structures, the principal one in the study area being the outlet of the Chain of Rocks Canal. In the pooled reach of the Upper Mississippi, impoundment has had a major influence on shoaling, with 80% of dredged volume required in the upstream halves of the pools. Although past engineering modifications have reduced sediment aggradation in the Mississippi channel, structural solutions to continuing shoaling can adversely impact the river's ecology, flood response, or hydrological function, and many structural solutions appear to be approaching their effective limits. The exception to this may be at highly localized shoaling locations, such as the Chain of Rocks Canal outlet, where flaws in the original design may still be remedied by structural means Targeted structural solutions may be effective at solving limited local shoaling problems, but we suggest that more sweeping new engineering modifications to reduce or eliminate dredging requirements are likely to be ineffective and/or outweighed by their adverse effects on the river system.     

A new look at wolf sunspot numbers in the late 1700's

Long-term homogeneous observations of solar activity or many solar cycles are essential for investigating many problems in solar physics and climatology. The one key parameter used in most long-term studies is the Wolf sunspot number, which is susceptible to observer bias, particularly because it is highly sensitive to the observer's ability to see the smallest sunspots. In this paper we show how the Wolf sunspot number can be derived from the number of sunspot groups alone. We utilize this approach to obtain a Group Wolf number. This technique has advantages over the classical method of determining the Wolf number because corrections for observer differences are reduced and long-term self-consistent time series can be developed. The level of activity can be calculated to an accuracy of ± 5% using this method. Applying the technique to Christian Horrebow's observations of solar cycles 1, 2, and 3 (1761–1777), we find that the standard Wolf numbers are nearly homogeneous with sunspot numbers measured from 1875 to 1976 except the peak of solar cycle 2 is too low by 30%. This result suggests that further analyses of early sunspot observations could lead to significant improvements in the uniformity of the measurements of solar activity. Such improvements could have important impacts upon our understanding of long-term variations in solar activity, such as the Gleissberg cycle, or secular variations in the Earth's climate.     

Organic nitrogen and caloric content of detritus III. Effect on growth of a deposit-feeding polychaete, Capitella capitata

Laboratory experiments showed that individual growth rate of the deposit-feeding polychaete, Capitella capitata (type 1) can be predicted by rate of organic nitrogen supply (ration) of detritus to the animals. The rate at which detritus is available to a deposit-feeder, whether by sedimentation to the bottom, or decomposition of otherwise undigestable components via microbial activity, or production rate of microbes themselves, is important and not just the concentrations of limiting substrate of different detritus sources. Even so, the portion of total caloric content of the detritus that can be utilized by a deposit-feeder—approximated by the ‘available’ caloric content—also contributes to limiting growth. However, available caloric content usually is low when nitrogen content is low, as in the case of vascular plant detritus. Trophic transfer efficiency (net production/food supplied) is a measurement of food chain transfer that is usually calculated in carbon or caloric units. If one is interested in comparing energy or carbon flow in food chains, then use of these parameters is appropriate; if one is interested in comparing the efficiencies of different species, then calculations must be made using nutritional factors limiting growth.