A Trophic Model for Tongoy Bay - a System Exposed to Suspended Scallop Culture (Northern Chile)
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JOURNAL OF EXPERIMENTAL MARINE BIOLOGY Journal of Experimental Marine Biology and Ecology AND ECOLOGY ELSEVIER 182 (1994) 149-168 A trophic model for Tongoy Bay - a system exposed to suspended scallop culture (Northern Chile) Matthias Wolff Zentrum fir Marine Tropeniikologie. Klagenfurter Str. GEO, 28359 Bremen, Germany Received 1 November 1993; revision received 14 March 1994; accepted 25 March 1994 Abstract A steady-state model of 17 compartments was constructed for the Tongoy Bay ecosystem using the ECOPATH II software of Christensen & Pauly (1992). The system is driven by planktonic production which is governed by periodical intrusions of upwelling water from a nearby upwelling centre. Of the total system biomass (236.3 g/m’), 47% is comprised of benthic invertebrates whose food intake exceeds that of pelagic fish and birds. Scallops in hanging cultures account for 7104 of the biomass in the water column. The model suggests that benthic invertbrate predators are more important in the cycling of biomass than demersal fish. A significant part of the production of the groups Macrophytes, Zooplankton, Phytoplankton, Suspended Scallops and a minor portion of other groups enters the detritus pool, from which an important amount of biomass re-enters the filter feeder compartments through bacteria. Global system properties such as total system through put(T), development capacity (C), ascendency (A) (sensu Ulanowicz & Mann, 1981; Ulanowicz, 1986; Baird & Ulanowicz, 1993) were also calculated by the means of net work analysis implemented in the ECOPATH II software. Results indicate that Tongoy Bay is an intermediate sized system (in terms of the sum of flows, T) of low maturity and high capacity to withstand ecological perturbations. The mean transfer efficiency between trophic levels is 14.7x, the fishery’s gross efficiency (primary production/catch and harvest) is 0.897”. In terms of global system properties these results are similar to those reported by Jarre et al. (1991) for the “open” Peruvian upwelling system during the peak period of anchovy catches (1963-1969). However, the mean trophic level of the fishery (3.6 for Tongoy Bay compared to 2.2 for the open upwelling system) and the biomass pathways differ significantly between these systems. Manipulation of the input data suggest that the bay has a potential for the production of biomass of filter feeders that is 10 to 15 times higher than the present production. Keywords: Biomass budget; Ecosystem structure; Scallop culture; Upwelling 0022-098 l/94/$7.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0022-0981(94)00053-G 1. introduction Tongoy Bay is a semi-enclosed shallow water embasement of about 60 km’ surface area located in Northern Chile (30” 14’30’s; 71 O30’ W). 40 km south of Coquimbo (Fig. 1). It is an important centre for tourism, fishing and invertebrate collection. Along its long beach (13.5 km) surf clams (Mesodesma donaciumn) are harvested intertidally by local fishermen and -women; subtidally an invertebrate diving fishery is operating. Its main target is the scallop (Argopectenpuvpurutus), whose largest banks of Chile are located here, but crabs (Cancerpol’odon) and snails (Xunthochonts sp.) are also col- lected. About 50”; of the pelagic and demersal fish landed in the fishing port of Tongoy is caught within the bay. Over the past years, scallop hanging cultures have increasingly become important. At present, several million scallops are suspended in the water column. Although some seed is artificially produced in hatcheries in Chile, most seed still comes from spat collectors or from the natural banks of the bay, were juveniles are clandestinely col- lected. The input of “unnatural” amounts of biomass through the scallop cultures into the system is most likely to significantly alter the biomass structure and trophic flows of the ecosystem and the question about the sustainability of the cultures and fisheries must be asked. While research has been done on the oceanographical conditions of the bay (Olivares et al., 1979; Olivares & Moraga, 1988; Acufia et al., 1989), and on most of its biotic compartments: plankton (AcuiIa et al., 1989; Alcayaga, 1990; Richter, 1990); scallop populations and other subtidal epibenthic macrofauna (WoIff & Alarcon, 1994); intertidal surfclam (Mesodesma donacium) populations (Alarcon, 1979) etc. no attempt has as yet been made to integrate the available information into a quantitative ecosystem model and to understand the main energy pathways of the system. Such an attempt is presented here, which was motivated by the following questions: (1) How is the biomass and biomass flow structure of this scallop dominated bay? (2) What are the main benthic predators, their prey items and consumption rates? (3) What is the amount of suspended scallop biomass compared to other filter feeders of the system? (4) What is the carrying capacity of the bay in terms of available food for filter feeders? (5) Which are the compartments most likely affected by the scallop cultures? (6) How do the energy fluxes within this bay differ from the open upwelling system? To model the bay’s ecosystem, the ECOPATH II software of Christensen & Pauly (1992) was used, which combines an approach of Polovina (1984) for estimation of biomass and food con- sumption of the various ecosystem elements (species or species groups) with an approach proposed by Ulanowicz (1986) for analysis of flows between the elements of an ecosystem and for the calculation of ecosystem indices. Among these are “Total System Throughput (T)“, which reflects the size of the system in terms of the sum of flows through all the individual compartments, “Ascendency (A)” which represents both the size and the organization of the flows and the “Development capacity (C)” which is the upper limit to ascendency. The degree of a system’s realized growth, organization and development can be given by the A/C ratio (Ulanowicz & Mann, 1981). Highly organized systems have the tendency to internalize most of their activ- ity and exchanges and become relatively independent of external inflows and outflows Fig. 1. Tongoy Bay. The upwelling centre ““Pta. Lengua de vaca” is located in front of Pta. Lengua de vaca. (Baird & Ulanowicz, 1993). The A/C ratio appears to be high in well organized sys- tems possessing significant internal stability which makes it difficult for new influences to change its basic structure, and lower in systems under stress. These indices have thus been used to compare ecosystems of different sizes, geographical location and com- plexity (Baird et al., 1991; Ulanowicz & Wulff, 1991). In the ECOPATH II model, biomass production of and imports to thecompartments is balanced by consumption and exports. Important input parameters for the model are: biomass (B), production per unit of biomass (PB), consumption per unit of biomass (QB), ecotrophic efficiency (EE) - the fraction of the production used in the system - and export (EX). Respiration (R), respiration per unit of biomass (R/B) and gross efficiency (GE) are output parameters that are crucial for examining the modelling results. The modeller defines the modet structure by a prey-predator matrix indicating the fractions of the total consumption coming from each prey source. 2. Material and methods 2. I. The study area The bay is influenced by a nearby upwelling centre (“lengua de vaca”, Fig. 1) which provides periodical intrusions of nutrient rich upwelling water (Acuda et al., 1989). Strong daily winds in the afternoon maintain a high water circulation. Water tempera- tures range from about 9 ‘C in the bottom waters in winter to 19 “C in the surface waters in summer (annual average: 14.6 “C). Under summer conditions of high radia- tion and weak winds, a thermocline develops up at about 10-l 5 m water depth, separating warm surface water ( 16- 19 ‘C) from colder bottom water (12- 15 ‘C). Oxy- gen concentrations usually exceed 409; saturation even in bottom water above 25 m. The deepest part of the bay measures 90 m, the average depth is around 25 m. In the lower parts of the bay (> 30 m) anoxic conditions occasionally exist during the late summer months. About 70?,, of the bay’s sediment is made of fine sand (average grain size 288 p) with an organic content ranging from 0. l- 1.09, (Pacheco et al.. 1987) but gravel bottom and sand mixed with shell debris can also be found as well as areas with stones (Wolff & Alarcon, 1994). These harder bottom areas are partly covered with macrophytes. Wolff & Alarcon (unpubl.) estimated an average macrophyte biomass in the bay of 25 g . m ’ (wet weight). In the northern sandy part of the bay small eel- grass beds extend over an area of = 3 ha. Nitrate, nitrite and phosphate concentrations increase with depth. In the depth range of 20 to 25 m they were measured as 10.5,0.38 and 1.42 pg at/l, respectively (Pacheco et al., 1987). 2.2. Basic modeling approach The core routine of ECOPATH II basically consists of using a set of simultaneous linear equations (one for each group i in the system), i.e.: Production by i-, all predation on i-, non-predation losses of i-. export of (i) = 0. for all i, M. Wolff/ .I. Exp. Mar. Biol. Ecol. 182 (1994) 149-168 153 or: Pi - BiM2i - Pi(l - EE,) - EX, = 0, (1) where Pi = the production of (i); Bi = the biomass of(i); M2i = the predation mortality of (i); EEi = the Ecotrophic Efficiency of (i); l-EE, = the “other mortality”; EX, = the export of (i). Thus, the biomass production (Pi = Bi.PBi) is the amount available to the system. Most of it will be used by predation (Bi . M2,), but a certain amount might be lost through other mortality [Pi(l - EE,)] or as export to other systems (EX,), i.e.