MEPS 613:15-47 (2019)  -  DOI: https://doi.org/10.3354/meps12825

ABSTRACT: The food webs of the 3 main reaches of the Ems estuary were analysed for 1975-1980 using Ecological Network Analysis (ENA). Aspects related to model aggregation at a high species resolution level (ALL SPP) or using functional groups (F-GR) and at 4 spatial scales (3 reaches and entire system) are cast in 24 balanced carbon flow models with 20 to 57 ENA compartments. Highest biomasses are represented by true phytoplankton, resuspended microphytobenthos, microphytobenthos, benthic bacteria, dissolved and particulate organic carbon. The bivalves Limecola balthica and Mya arenaria represent the highest animal biomass, while Crangon crangon plays a central role in the network connections. In an upstream direction, there are clear gradients in the relative overhead (Φ/DC: 0.610-0.599-0.534 [values for the 3 reaches]) and the relative ascendency (A/DC: 0.390-0.401-0.466). The A/DC values decrease whenthe non-living compartments (detritus) as well as the living compartments (species, functional groups) are hand-balanced. The A/DC values increase when the models are balanced by the available AVG2 balancing routine. The Φ/DC values show opposite trends. For the ALL SPP models, the geometric mean trophic efficiencies (MTE) decrease in an upstream direction (2.71-1.01-0.89) while the Finn cycling indices (FCI) increase (10-8.4-16.6%), as do the detritivory/herbivory ratios (3.13-4.33-9.03). Application of ENA to reaches instead of a full system is therefore suggested. Since ENA is sensitive to relatively small quantitative changes in biomass input values, the preferred user protocol is to consider all the relevant species separately, and to hand-balance the flows of living as well as non-living compartments.

KEY WORDS: ENA indices · Assessment · Ecosystem · Indicators · enaR