Technical note: The effects of five different defaunation methods on biogeochemical properties of intertidal sediment

Various methods have been used to remove organisms from sediments to investigate structure and function of faunal assemblages in intertidal habitats. Nevertheless, little is known about how these treatments affect properties of the sediments themselves, although changing these properties may cause changes in the assemblages, independently of other hypotheses being tested. This study assesses the efficacy of defaunation and effect on selected biogeochemical properties of five different methods of defaunating soft muddy sediments in an estuary. The methods were removal and freezing of sediment, removal and oven-heating, freezing in situ with liquid N2, spraying with formalin and spraying with hydrogen peroxide. The first four of these methods have been used in previous studies, whilst the fifth was considered to be a potentially useful defaunator because it does not leave toxic residues. The first two methods required sediment to be brought back to the lab, disrupting the natural structure of the sediment; the last three were done in situ, with much less disturbance.  Variables measured to assess effects of the treatments on the sediment were amount of water, grain size, total carbohydrate, suspension index (relative erosion rate), erosion threshold, chlorophyll a and b, colloidal carbohydrate, Fo (minimal fluorescence) and Fv/ Fm (photosynthetic yield). There were no significant effects of any treatment on the first four variables. For the others, effects of defaunation varied from treatment to treatment and with time after treatment. Generally, the greatest disturbance was to the microphytobenthos (MPB, measured by chlorophyll and fluorescence) and related variables. For most treatments, recovery was rapid, but the effects of formalin and H2O2 persisted for a few days. Effects on physical properties of the sediment were mostly minor and insignificant. Removal and freezing or heating, however, caused major changes to the sediments because of the disturbances involved. Choosing the appropriate method of defaunation is very important if interpretations are not to be confounded between the effects of defaunation per se and any effects of changes to other biota (such as microphytobenthos) and/or the properties of sediments caused by the method used to defaunate experimental areas

Destabilization of cohesive intertidal sediments by infauna

Bioturbation activity was reduced in four plots on an intertidal mudflat in the Humber estuary (UK) during 4 days, by spraying the sediment with an insecticide, namely vydate. Macrofaunal, especially Nereis diversicolor and oligochaeta, and meiofaunal densities decreased, while the diatom biomass did not change. This resulted in a 300% increase in sediment stability, caused by a reduction in bioturbation and grazing pressure and a decrease in the water content [KEYWORDS: sediment stability, benthos, erosion, intertidal flats, Humber]

The effects of tidally driven temporal variation on measuring intertidal cohesive sediment erosion threshold

Accurate measurement of intertidal sediment erodibility is essential for the development of meaningful and accurate models of sediment dynamics. Despite considerable advances in technology and methodology, the measurement of cohesive intertidal sediment erosion remains problematic. Sediment erodibility varies according to both physical and biological properties and processes. These cannot be considered in isolation, as they can interact to create both positive and negative feedbacks, resulting in seemingly idiosyncratic responses in the system. If working models of estuarine sediment dynamics are to be made, it is essential that the influence of these processes on both the measurements and the system itself be considered. Recent developments in measurement technology enable rapid measurement of sediment stability allowing temporal and spatial variability to be measured on a time scale of minutes. This paper reports temporal variability in cohesive intertidal sediment erosion threshold related to immersion and emersion, and the concomitant responses in selected sediment properties (carbohydrates, water content and chlorophyll). Erosion threshold tended to increase over emersion and decrease over immersion, although the patterns of change varied depending upon local conditions, and in one case there was no temporal trend. Temporal changes resulted in a range of measured erosion threshold, dependant upon the erosion device used. Modifications to existing methodology, in order to account for this variation, are proposed and implications for modelling erosion processes are considered.</p

The effects of rain on the erosion threshold of intertidal cohesive sediments

Intertidal sedimentary environments are complex systems governed by interactions between physical, chemical and biological processes and parameters. Tidally induced flow and wave action are known to be an integral driving force behind the erosion, transport, deposition and consolidation cycle (ETDC) of intertidal sediments. Whilst considerable advances have been made in understanding both the physical and biological processes and their interactions in these systems, it is clear that there are gaps in our understanding. One factor that has been largely ignored to date is that of rain. Visual observations in the field and associated data indicated that rain showers during low tide are correlated with a reduction in the erosion threshold of intertidal cohesive sediments. This paper presents preliminary field and laboratory data showing the importance of rain in reducing the erosion threshold of cohesive intertidal sediments. The implications for our knowledge of, and modelling of the ETDC cycle of cohesive intertidal sediments are discussed

In site versus laboratory analysis of sediment stability from intertidal mudflats

Also published in: Continental Shelf Research 20 (2000), 1317-1334SIGLEAvailable from TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman