Detritus: Mother Natures Rice Cake Pamela Mason and Lyle Varnell Detritus has been studied since ancient times. gested that man should devise ways to use de- Early man and ancient civilizations considered tritus as food (Odum 1969). muds and slimes the source and sustenance of all aquatic life (Darnell 1967). Early civilizations The importance of detritus as the basis of eco- which relied on the bounties of the sea for nutri- system foodwebs has been researched exten- tion and commerce, and which worshipped ele- sively. In estuarine foodwebs particularly, sci- ments within the entific studies indi- oceans, highly cate that detritus revered detritus. serves as a food source for micro- While todays scopic organisms, popular opinion which in turn are con- appears to sim- sumed by larger or- ply discount the ganisms, forming the importance of de- basis for the estua- tritus as just rine foodweb. As will plain mud, sci- be discussed in this entific evidence paper, and a second, indicates that more detailed paper the ancient cul- to follow, the estua- tures may have rine detrital foodweb been correct con- is fairly complex, yet cerning the im- fundamentally based portance of detri- on the decay of veg- tus. Detritus is etative matter provid- important on a ing a growing sub- global scale as strate for microbes well as locally; which are a food from its role in source for larger ani- the world carbon mals. In turn, these cycle to supplying animals are prey for part of the nutri- larger animals and tional require- eventually, recreation- ments of a ally and commercially marsh peri- important finfish, winkle (Stumm shellfish, crusta- & Morgan 1981, ceans, waterfowl and Baker & Allen wading birds. 1977). It has even been sug- 1 What is Detritus? be limited to the organic matter, the material which is being broken down, and should exclude Detritus, derived from the Latin root meaning to the microorganisms which are the agents of break down or wear away, is used to define or- breaking down the substrate. However, a more ganic muds, slimes and oozes. Early efforts by functional definition of detritus includes atten- the scientific community to define detritus re- dant microorganisms with the organic debris. sulted in very general definitions. Darnell (1961) As much of the current interest in detritus re- defined detritus as all types of biogenic mate- lates to the role it plays in marine foodwebs, rial in various stages of bacterial decomposition. and as microorganisms are a fundamental ele- Later, Darnell (1967) modified his definition to ment of that role, we will use the functional defi- all types of biogenic material in various stages nition which includes attendant microorgan- of microbial decomposition which represent po- isms. tential energy sources for consumer species. However, further efforts by the scientific com- Using any definition, detritus is found in all munity to define detritus were hampered by the biotopes. This report will address the marine philosophical debate over the inclusion, or ex- environment only, and specifically, the estuarine clusion, of microorganisms. In one school of and marsh environment. thought, detritus refers to organic debris to- gether with attendant microorganisms, while another school of thought defines the substance What is in Detritus: Composition as detritus plus attendant microorganisms Detritus can be composed of a diverse suite of (Crosby 1985). In other words, is detritus the living and non-living constituents. The gross non-living organic debris (vegetative matter and composition of marine detritus includes particu- feces) alone, or does it include the living micro- late vascular plant matter, Monerans (blue-green organisms? Classically, the definition should algae and bacteria), yeasts and other fungi, al- gae (benthic and phytoplankton), carcasses, fe- ces, bacterial exopolymers, protozoans (ciliates and zooflagellates), metazoans (nematodes, tur- Wetlands Program bellarians, rotifers, ostracods, and harp- School of Marine Science acticoids), shed exoskeletons, regurgitations, Virginia Institute of Marine Science molecular aggregates (colloidal lipids, carbohy- College of William and Mary drates and proteins), dissolved liquids (vita- Gloucester Point, Virginia 23062 mins, amino acids, simple sugars, and urea), and dissolved gases (methane, ammonia, and Dr. Carl Hershner, Program Director hydrogen sulfide) (various authors). Mr. Thomas A. Barnard, Jr., Editor Dr. Kirk Havens, Editor Fecal matter is a relatively important component Published by: VIMS Publication Center of detritus. Fecal pellets are a major marine food source which is high in protein, and ingestion This technical report was funded, in part, by the and use of such is an important marine energy Virginia Coastal Resources Management Program of transfer mechanism (Frankenberg & Smith 1967, the Dept. of Environmental Quality through Grant Johannes & Satomi 1967). #NA57OZ0561-01 of the National Oceanic and At- mospheric Administration, Office of Ocean and Coastal Resources Manage- Where Does Detritus Come From: ment, under the Coastal Zone Manage- Sources ment Act of 1972, as amended. Natural sources of marine detritus include ani- The views expressed herein are those of the mals, phytoplankton, bacteria and blue green authors and do not necessarily reflect the views of algae, periphyton, submerged aquatic vegeta- NOAA or any of its subagencies or DEQ. tion, intertidal macrophytes, river borne detri- tus, beach and shore material, terrestrial detri- Commonwealths Declared Policy: to preserve the wet- tus and atmospheric deposition (Darnell 1967 lands and to prevent their despoliation and destruction... and others). Detritus is produced during all sea- sons and is available in three forms: particu- Printed on recycled paper. late, micellar (organic material adsorbed onto 2 inorganic particles), and dissolved (Darnell proximately equal to phytoplankton (770 g m2y1) 1961, Odum & de la Cruz 1967). Particulate and terrestrial (600 g m-2y-1) sources for a Geor- forms can either be suspended in the water col- gia salt marsh. umn or precipitated. The sources of particulate detritus can be determined by chemical testing, as sources have distinct signatures. The im- How is Detritus Produced? portance of each source varies with area, salin- ity and season. There is little scientific information on the decay rate of salt marsh vegetation. However, some The great bulk of organic detritus is derived from research has been undertaken to study the pro- vegetation. The majority of vegetative material duction of detritus from Spartina alterniflora. in salt marshes is of macrophyte origin. Since approximately 2/3 of the primary produc- Spartina alterniflora and algal species are the tion of east coast intertidal marshes is derived two major sources of primary production in salt from Spartina alterniflora (Pomeroy et al. 1981), marshes, but algal production is estimated to a discussion of these processes is particularly be only 20-25% of relevant. marsh grass pro- Most halophytes, duction. Esti- Ash Free Dry Weight is the determination of mass af- Spartina alterniflora mates of aerial ter the removal of water and carbonaceous material. It included, are physi- production for is commonly used to determine the mass of the struc- ologically patterned Spartina alterni- tural components of living matter, or the organic car- much like terrestrial flora range from bon content of soils. There are several techniques by plants (Haines & 550-2,000 grams which this determination may be made. In the most Montague 1979). per meter square simple technique, a sample is ignited (burned) at high -2 -1 Therefore, marsh per year (g m y ) temperature. Prior to ignition, the material is placed in grass detritus is com- dry weight with a low temperature oven, commonly referred to as a dry- posed primarily of below-ground ing oven, to remove the water content of the material. structural plant production up to The material is then weighed (dry weight) and placed in o polymers such as cel- 7 times aerial a high temperature (550 C) muffle furnace for a couple lulose, hemicellu- production (Mar- hours. At this temperature the organic material is lose and lignin. inucci 1982). burned off and the structural material remains. With These polymers are This far exceeds vegetative samples, the ash residue is calculated, and collectively referred to marsh phyto- in soils the loss-on-ignition (the difference between the as lignocellulose plankton or ben- dry weight and ash free dry weight) provides a rough (Benner et al. 1988, thic algae produc- estimate of organic carbon. (Allen et al. 1986) tion. However, Hodson et al. 1984). there is an in- Lignocellulose consti- crease in benthic algae production during win- tutes from 55-80% of the ash free dry weight of above-ground Spartina alterniflora biomass ter months when the senescence of the marsh grasses allows light to penetrate a greater area (Coston-Clements & Ferguson 1985, Benner et of the marsh surface (Gallagher & Daiber 1974, al. 1988). (See box above.) Lignin, starch and Peterson & Howarth 1987, Van Raalte et al. 1976, lipids make up the remaining percentage of the Benner et al. 1988). above-ground biomass, but can make up a greater percentage of ash free dry weight than Mann (1976) estimates that a maximum of 10% either cellulose or hemicellulose in the below- of marine macrophyte (both emergent and sub- ground portion of Spartina alterniflora (Coston- merged aquatic) production is directly consumed Clements & Ferguson 1985, Newell & Langdon by primary consumers and the remaining 90%, 1986). Spartina alterniflora lignocellulose, by or more, enters detritus food chains. Studies in weight, is approximately 41% carbon and <1% a Georgia estuary estimate that of the total de- nitrogen (Benner et al. 1988). trital pool, approximately 86% of the detritus was decaying Spartina alterniflora (Odum & de Valiela et al. (1985) showed decay rate to be di- la Cruz 1967). Hodson et al. (1984) showed rectly related to internal nitrogen levels of the annual detrital production from Spartina decaying material, generally the higher the nitro- alterniflora to be approximately 1,200 g m-2y-1.