Forest Investment Account (FIA) - Forest Science Program
FIA Project Y092261

    Biogeochemical indicator and threshold for assessing ecological impacts of riparian forest management on downstream ecosystems
Project lead: Sakamaki, Takashi (University of British Columbia)
Contributing Authors: Sakamaki, Takashi; Shum, Jennifer Y.T.; Richardson, John S.
Subject: Forest Investment Account (FIA), British Columbia
Series: Forest Investment Account (FIA) - Forest Science Program
Riparian forest management substantially affects transport of various materials (e.g. sediment, organic matter, nutrients) through river systems to estuaries [1-6]. These materials have a potential cumulative effect on the physical and chemical structure of habitats and food webs in downstream ecosystems. In the BC coast area, there are many steep mountainous streams, and some of them transport substantial amounts of terrestrial organic materials to their estuaries [7, 8]. However, it is unclear how forestry activities in riparian areas modify downstream ecosystems and how far along streams they affect, because specific indicators and thresholds have not been established to coherently assess effects of forest management on downstream ecosystems. The development of these indicators and thresholds will enhance forest management schemes. To date, although it has been shown that forestry activities in riparian area substantially modify terrestrial material inputs, food webs and biota in its adjacent stream [9-13], fate of fluvial materials exported from such sites and its subsequent effects on downstream ecosystems have not been tracked. The objectives of this project are to (1) analyse longitudinal transports of particulate organic matter (POM) exported from upstream riparian forest management sites, (2) analyse biological effects of the exported POM on downstream ecosystems including estuaries, and (3) propose biogeochemical indicators and thresholds to assess effects of upstream riparian forest management on downstream ecosystems. In this proposal, “downstream” is defined as the reach below which forestry activity has been operated including the estuary, while “upstream” includes the forestry activity site. The biogeochemical indicators will be based mainly on quantity and quality of POM in downstream systems as well as at riparian forestry management sites. The threshold will define levels of the indicators above/below which food webs or biota of downstream systems are significantly altered. Our literature review suggests that to develop such tools, two scientific issues need to be overcome: how to track longitudinal transport of upstream-origin POM (UPOM), and what to consider as factors controlling UPOM effects on downstream ecosystems. In other words, without solving them, it cannot be tested whether upstream forestry management affects downstream ecosystems.
Stable isotopes (SI) of carbon (C) and nitrogen (N) and some other elements are very commonly used to differentiate ‘autochthonous and allochthonous’ organic materials in various aquatic systems. SI signatures probably have the most potential as tracers to track UPOM in downstream systems. However, algal production and terrestrial material input to stream systems alter the relative contribution of UPOM to downstream POM [14-17], and potentially complicate detection of UPOM in downstream systems by SI signatures. Thus, it is difficult to determine spatial origins and historical records of downstream POM only by SI signatures. In previous studies of river, d13C of fine POM (FPOM) showed simple increasing trends from upstream to downstream as primary productivity increasing [17]. This suggests that SI of FPOM was dominantly controlled by the balance of fluvially transported FPOM and FPOM from primarily producers, and that budgetary analysis along stream systems could be effective to examine FPOM transport in downstream systems. In a budgetary approach, however, quantifications of various fluxes (e.g. deposition, resuspension, terrestrial POM input, primary production) generally complicate procedures. In stream systems with relatively steep gradient, which dominate the BC southwest coast and are targeted in this project, although FPOM is thought to flow downstream quickly [18], the relative importance of the processes for FPOM budget is still unknown. Both SI and budgetary approaches have their own advantages, therefore, they will be applied together in this project. Analyses of FPOM for SI as well as other biogeochemical signatures (e.g. C, N, Chl.a contents) can differentiate the sources and also generate as many budgetary equations as the number of unknown fluxes. These equations will be solved simultaneously to estimate dilution and longitudinal transport of UPOM in downstream systems.
Meanwhile, since coarse POM (CPOM) (e.g. leaf, wood) has spatially patchy distributions in ecosystems, clarification of its dynamics probably necessitates another strategy. It is known that leaf material gradually increases its nitrogen content at day to week scales in fresh- and blackish-waters due to nitrogen incorporation from ambient water by microbial activities (e.g. bacteria, fungi) [8, 19-22]. Since fluvial transport of POM and temporal change in nitrogen content have similar timescales [18], its nitrogen content could be a useful signature to estimate leaf retention in aquatic systems. We will apply this to determine CPOM dynamics in downstream systems.
Our study has shown that substantial amount of terrestrial origin POM accumulated in the sediment in an estuary of the BC southwest coast which connected to a steep mountainous stream [7]. In estuaries connected to gradual and eutrophic streams, however, river POM dominated by algal material was negligible in the sediment [7]. In addition, leaves of deciduous trees transported as CPOM from a mountainous stream provided estuarine invertebrates with microhabitats [8]. Thus, to assess responses of downstream ecosystems to UPOM, unique aspects of UPOM need to be focused on; in particular, terrestrial material is more refractory than aquatically-produced organic matter, e.g. algae, and contains a lot of coarse fractions, e.g. leaves and wood debris. Furthermore, algal material has higher nutritional value than terrestrial POM and can be a preferable food source for secondary producers, which has been reported in both riverine and estuarine ecosystems [7, 11, 23-25]. However, terrestrial POM can enhance its nutritional value for secondary producers with time due to bacteria and/or fungi absorbing nitrogen from ambient water [7, 19, 26, 27]. Particularly in downstream systems including estuaries, since terrestrial material has longer residence time, their contribution to secondary production is likely more possible. In fact, contributions of terrestrial organic matter to the food web have also been reported [7, 27-30]. Overall, it is still unclear which factors more significantly control the incorporation of UPOM into recipient food webs. Thus, in this project, contribution of UPOM to food webs in downstream ecosystems will be assessed testing the effect of two major factors; quality of UPOM (e.g. C:N, algal content), and characteristics of recipient systems (e.g. primary productivity, retention of POM). This is expected to generalise UPOM effects on downstream food webs, and give a new insight to general interpretation on subsidy effect in ecosystems.
Related projects:  FSP_Y081261FSP_Y103261

Executive summary (0.3Mb)
Abstract (61Kb)
Abstract 2 (49Kb)

Updated August 16, 2010 

Search for other  FIA reports or other Ministry of Forests and Range publications.

Please direct questions or comments regarding publications to