Quantification of the Synergistic Effects of Eutrophication, Apex Predator Pressure, and Internal Processes on the Black Sea Ecosystem

Year 2013, Volume: 13 Issue: 4, - , 01.08.2013

Baris Salihoglu Nusret Sevinc

https://doi.org/10.4194/1303-2712-v13_4_03

Abstract

In the present study, a model of the lower-trophic pelagic food web of the Black Sea is considered in order to provide a quantitative understanding of the marked changes in the food web structure in response to changing top-down and bottom-up control mechanisms due to eutrophication, overfishing, and climatic changes. The simulations consider three particular parameters controlling the changes in the ecosystem structure due to these stressors; (i) the magnitude of the nitrate flux into the euphotic layer from the chemocline layer (enrichment due to eutrophication), (ii) the magnitude of predation control introduced by the planktivorous fish on mesozooplankton (fishery), (iii) the magnitude of the Q parameter controlling temperature dependence of the Mnemiopsis growth (changing climatic conditions). A simulation assessing the level of enrichment on the ecosystem indicates a shift of the major trophic energy flow towards Noctiluca and Aurelia and thus showing how the ecosystem would degrade when a critical level of enrichment is passed. Increasing predation pressure of the planktivorous fish during the enrichment phase is shown to reduce the mesozooplankton biomass that in turn declines its predation pressure on Noctiluca population and thus allows Noctiluca biomass to increase, considering that mesozooplanton is assumed to feed on Noctiluca (at a level of 15% food preference) in the current model.. Noctiluca then acts as a major consumer and a critical element of the degraded food web structure. The overfishing of planktivorous fish stocks allows Mnemiopsis biomass to increase considerably under changing (warmer) climatic conditions at the end of 1980s, following a decade-long intense cooling phase. Thus, our results suggest that overfishing would not alone be able to promote a Mnemiopsis population outburst in the absence of warming.

Keywords

Black Sea, population dynamics, trophic levels, nutrient enrichment, predation control, mnemiopsis.

Quantification of the Synergistic Effects of Eutrophication, Apex Predator Pressure, and Internal Processes on the Black Sea Ecosystem

Abstract

Karadeniz besin ağında gözlemlenen değişimlerin ötrofikasyon, balıkçılık ve iklim değişikliklerinden kaynaklı üstten alta (top down) ve altan üste (bottom up) kontrol mekanizmalarına verdiği tepkiyi anlayabilmek üzere bir Karadeniz besin ağı modeli kullanılmıştır. Gerçekleştirilen simülasyonlarda bu baskıları kontrol eden üç parametre seçilmiştir; (i) kemoklin tabakadan ışıklı tabakaya (öfotik bölge) olan azot girdisi (ötrofikasyon kaynaklı), (ii) planktivor balıklar tarafından mesozooplankton üzerinde uygulanan predasyon baskısı (balıkçılık etkisi), (iii) Q parametresinin Mnemiopsis büyümesini sıcaklığa göre kontrolü (iklim etkisi). Besin tuzu artışının ekosistem üzerindeki etkisinin analizine yönelik yapılan simulasyon sonuçlarına göre bu koşullar altında trofik enerji akışı Noctiluca ve Aurelia yönünde olmuştur. Yine bu dönemde artan planktivor balık kaynaklı predasyon baskısı mesozooplankton biyokütlesinin azalmasına yol açmış ve bu da Noctiluca üzerindeki trofik baskıyı azaltarak Noctiluca populasyonunun artmasında rol oynamıştır. Burada mevcut modelde mesozooplanktonun Noctiluca üzerinde azda olsa (%15) beslenebildiği unutulmamalıdır. Bunu takiben Noctiluca besin ağı ve trofik seviyeler arası enerji akışı içerisinde etkili bir konuma gelmiştir. Planktivor balıkların aşırı avlanması sonucunda 1980’lerin sonunda on yıllık soğuk dönemden sonra değişen iklim koşullarıyla beraber (artan sıcaklık) Mnemiopsis biyokütlesi önemli ölçüde artmıştır. Dolayısıyla sonuçlar, aşırı balıkçılığın Mnemiopsis biyokütlesinde görülen aşırı artışı artan deniz suyu sıcaklığının etkisi olmaksızın tetikleyemeyeceğine işaret etmektedir.

Keywords

Black Sea, population dynamics, trophic levels, nutrient enrichment, predation control, mnemiopsis.

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