Exploring effects of dredging and organic waste on the functioning and the quantitative biomass structure of the Ems estuary food web by applying Input Method balancing in Ecological Network Analysis

Abstract

Detailed food webs of the three main Ems estuary reaches (Lower Reaches/LR, Middle Reaches/MR and Dollard/DO) representing the 1975–1980 period, were used for explorations. The effects of turbidity and organic waste were explored for ∼1955 (waste loads and low suspended matter) and ∼2005 (no waste loads and high suspended matter) by applying the Input Method-based balancing as part of the Ecological Network Analysis (ENA) package to analyse food web carbon flows. The total phytoplankton primary production (sum of true phytoplankton and resuspended microphytobenthos) of the LR in 2005 is reduced by 56% compared to 1955. For MR and DO the primary production reduction over the same period is ∼70%. Expressed in tons carbon the 2005 total primary production (including microphytobenthos) of the entire estuary is 59% of that in 1955. In 2005 the total (all living compartments) estuarine biomass is 63% of that in 1955. The data suggest a further fast future biomass decline. Compared to 1955 the relative biomass changes of the living compartments are in 2005 −71.1% for LR, −66.7% for MR and −70% for the Dollard. Results are in line with independent monitoring. The biomass changes per species range for LR from −1.3% (microzooplankton) and −1.5% (intertidal bivalves) to −71.7% (pelagic bacteria), for MR from −15.3% (mesozooplankton, Sprattus sprattus, Clupea harengus) to −90.5% (pelagic bacteria) and for DO from −6.6% (Chelon labrosus) to −97.4% (pelagic bacteria). The relative mean total biomass changes caused by the sanitation of the organic waste loads are −1.5% for LR, −5.0% for MR and −12.5% for DO. The IM-based balancing results in an increasing ratio between detritivory and herbivory and increase of the carbon cycling. The effects are strongest for the LR. The described changes are driven by i) the combination of changed system boundary conditions and the species specific diets and ii) the absence of international political collaboration because of a disputed border that results in the absence of an internationally agreed integral solution via one master plan Ems estuary. The varying changes at the species level and the strong connection of pelagic and benthic systems in intertidal flat estuaries demand an integral approach for assessments and management.