Experimental Approaches to Sediment Communities
341
mudflats, also from the riverine catchment. Intertidal flats, therefore, provide
an ideal opportunity to explore the role (if any) that subsidies might play in
the dynamics of food webs. Intertidal flats have the additional advantage that,
unlike many systems, experimental manipulations can be carried out with
relative ease. In this section, we describe the results of a series of field experiments and other research on the Ythan estuary food web that are relevant to
the subsidy question. Specifically, we examine the hypothesis that subsidies
which increase the abundance of primary producers, invertebrate consumers
and their predators, have cascading effects on community organisation, as
postulated by Polis et al. (1997).
16.2.1 Effects of Subsidised Primary Producers
The most significant primary producers on the Ythan are the green macroalgae Enteromorpha, Ulva and Chaetomorpha, all of which seem to be
nitrogen-limited in this system (Raffaelli 1999). Most of the nitrogen entering
the system is imported to the mudflats by the river entering the estuary, being
ultimately derived from farmland. Nitrate concentrations arriving at the
estuary have increased three-fold (from about 3mg N0 3 1-1 to 8mg N0 3 1-1)
from the 1950s to date (Raffaelli 1999). The biomass and distribution of green
macro algae has similarly increased over the same period, so that 40 % of all
mudflats are now affected by algal mats with biomasses of 1-3 kg wet wt. m- 2 •
The links between land-use, nutrients and algal biomass are compelling and
we are confident that primary production in this system is subsidised by
nutrients from terrestrial sources (Raffaelli 1999).
But what effect does this subsidised macro algal biomass have on the
mudflat invertebrate assemblage? This issue has been explored through a
series of manipulative field experiments, where invertebrate assemblages
were compared under different levels of algal biomass (Hull 1987; Raffaelli
2000a; Fig. 16.1). In none of these experiments was there any evidence of an
increase in macro algal consumers. In other words, elevation of the resource
(algae) by a subsidy (nutrients) did not affect consumer-resource dynamics.
Indeed, densities of most invertebrates were dramatically reduced by
macro algal mats, due to the hostile redox environment and physical
interference with invertebrate-feeding mechanisms (Raffaelli et al. 1998;
Raffaelli 2000a). Only the opportunistic polychaete Capitella sp. consistently
increased in abundance under macro algal mats, presumably due to the
organically enriched sediments (Fig. 16.1). Thus, whilst enhanced biomass of
algae does have effects on invertebrate assemblages at the local scale, these are
mainly indirect, through changes in habitat quality rather than trophic
dynamics. However, the densities of deposit-feeding invertebrates have
increased in other sections of the estuary not affected by macro algal mats
341
mudflats, also from the riverine catchment. Intertidal flats, therefore, provide
an ideal opportunity to explore the role (if any) that subsidies might play in
the dynamics of food webs. Intertidal flats have the additional advantage that,
unlike many systems, experimental manipulations can be carried out with
relative ease. In this section, we describe the results of a series of field experiments and other research on the Ythan estuary food web that are relevant to
the subsidy question. Specifically, we examine the hypothesis that subsidies
which increase the abundance of primary producers, invertebrate consumers
and their predators, have cascading effects on community organisation, as
postulated by Polis et al. (1997).
16.2.1 Effects of Subsidised Primary Producers
The most significant primary producers on the Ythan are the green macroalgae Enteromorpha, Ulva and Chaetomorpha, all of which seem to be
nitrogen-limited in this system (Raffaelli 1999). Most of the nitrogen entering
the system is imported to the mudflats by the river entering the estuary, being
ultimately derived from farmland. Nitrate concentrations arriving at the
estuary have increased three-fold (from about 3mg N0 3 1-1 to 8mg N0 3 1-1)
from the 1950s to date (Raffaelli 1999). The biomass and distribution of green
macro algae has similarly increased over the same period, so that 40 % of all
mudflats are now affected by algal mats with biomasses of 1-3 kg wet wt. m- 2 •
The links between land-use, nutrients and algal biomass are compelling and
we are confident that primary production in this system is subsidised by
nutrients from terrestrial sources (Raffaelli 1999).
But what effect does this subsidised macro algal biomass have on the
mudflat invertebrate assemblage? This issue has been explored through a
series of manipulative field experiments, where invertebrate assemblages
were compared under different levels of algal biomass (Hull 1987; Raffaelli
2000a; Fig. 16.1). In none of these experiments was there any evidence of an
increase in macro algal consumers. In other words, elevation of the resource
(algae) by a subsidy (nutrients) did not affect consumer-resource dynamics.
Indeed, densities of most invertebrates were dramatically reduced by
macro algal mats, due to the hostile redox environment and physical
interference with invertebrate-feeding mechanisms (Raffaelli et al. 1998;
Raffaelli 2000a). Only the opportunistic polychaete Capitella sp. consistently
increased in abundance under macro algal mats, presumably due to the
organically enriched sediments (Fig. 16.1). Thus, whilst enhanced biomass of
algae does have effects on invertebrate assemblages at the local scale, these are
mainly indirect, through changes in habitat quality rather than trophic
dynamics. However, the densities of deposit-feeding invertebrates have
increased in other sections of the estuary not affected by macro algal mats