A decline in the density of zoobenthos has taken place in Lake Abiskojaure in the northern Swedish mountains between 1988 and 2019 but no response can be seen in the abundance (CPUE) of Arctic charr (Salvelinus alpinus) that feed mainly on benthic invertebrates. To evaluate if possible shifts in resource use of charr explain this lack of change in population abundance, stomach contents (i.e. diet) of charr between 1985 and 2020 were analyzed. Charr diets changed during the study period, with decreasing contributions to the diet of larger sized prey and increasing contribution of smaller sized prey and zooplankton. A corresponding recent decline in charr growth, size, and condition factor was observed. The proportion of large prey in diets and charr growth both show a hump-shaped relationship with time before the recent decline, indicating consumption of large prey had a positive effect on charr growth. Densities of zooplankton in the lake did not change with time, meaning increases in diet proportions are driven by something else. Declines in nitrogen and phosphorous over time may have contributed to the decline in zoobenthos densities through nutrient limitation of primary production. The results from this study suggest charr has changed diet towards smaller, less energetically efficient prey as a response to declining zoobenthos levels, and that this has had a negative effect on charr growth, size, and condition factor. Additional declines in nutrients and zoobenthos abundance may further worsen charr performance and eventually be manifested as declining charr abundance.

A decline in the density of zoobenthos has taken place in Lake Abiskojaure in the northern Swedish mountains between 1988 and 2019 but no response can be seen in the abundance (CPUE) of Arctic charr (Salvelinus alpinus) that feed mainly on benthic invertebrates. To evaluate if possible shifts in resource use of charr explain this lack of change in population abundance, stomach contents (i.e. diet) of charr between 1985 and 2020 were analyzed. Charr diets changed during the study period, with decreasing contributions to the diet of larger sized prey and increasing contribution of smaller sized prey and zooplankton. A corresponding recent decline in charr growth, size, and condition factor was observed. The proportion of large prey in diets and charr growth both show a hump-shaped relationship with time before the recent decline, indicating consumption of large prey had a positive effect on charr growth. Densities of zooplankton in the lake did not change with time, meaning increases in diet proportions are driven by something else. Declines in nitrogen and phosphorous over time may have contributed to the decline in zoobenthos densities through nutrient limitation of primary production. The results from this study suggest charr has changed diet towards smaller, less energetically efficient prey as a response to declining zoobenthos levels, and that this has had a negative effect on charr growth, size, and condition factor. Additional declines in nutrients and zoobenthos abundance may further worsen charr performance and eventually be manifested as declining charr abundance.

Ökosystem-Ingenieure sind Organismen, deren Auswirkung auf die Funktion von Ökosystemen im Vergleich zu ihrer Anzahl und Biomasse überproportional groß ist. Ein klassisches Beispiel für Ökosystem-Ingenieure sind grabende Organismen, deren Aktivitäten (Bioturbation) sowohl die Sedimentmatrix als auch das Porenwasser in aquatischen Sedimenten beeinflussen. Solche Tiere wirken auf eine große Anzahl von biogeochemischen Prozessen in benthischen Ökosystemen ein, unter anderem auf die aerobe Atmung (Respiration). Die Respiration aquatischer Sedimente umfasst häufig über 50 % der gesamten Respiration von aquatischen Systemen und spielt eine große Rolle im globalen Kohlenstoffkreislauf. Die vorliegende Doktorarbeit beschäftigt sich mit den Auswirkungen der physikalischen Umwelt (Sedimenteigenschaften, am meistens hydraulischer Leitfähigkeit) auf die mikrobielle Respiration von Sedimenten, in denen Bioturbation durch Chironomidenlarven stattfindet. Um die Auswirkungen von Bioturbation auf Respiration zu messen und zu identifizieren, wurde eine neue Messmethode entwickelt (Kapitel 4.1). Kapitel 4.2 zeigt, dass der Einfluss von Bioturbation auf die Respiration des Sediments mit zunehmender Temperatur ansteigt. Kapitel 4.3 belegt, dass Resazurin auch für die Messung der Respiration in marinen Sedimenten geeignet ist. Kapitel 4.4 vergleicht und begutachtet die große Anzahl und Vielfalt hydrologischer, biogeochemischer und ökologischer Tracer einschließlich Resazurin. Die physikalische Umwelt (Sedimentmatrix) kontrolliert wie stark die Auswirkungen der Bioturbation auf die Respiration des Sedimentes sind. Dementsprechend liefert diese Doktorarbeit die Basis für das Verständnis der Auswirkungen benthischer Bioturbation auf Respiration und Kohlenstoffumsatz in limnischen und marinen Sedimenten. / Ecosystem engineers are organisms, whose impact on ecosystem functioning is disproportionally large compared to their abundance and biomass. A classic example of ecosystem engineers are burrowing organisms whose activities (bioturbation) affect the sediment matrix and pore solutes in aquatic sediments. Bioturbating animals are impacting on a number of biogeochemical processes in benthic ecosystems, including, among others, aerobic respiration. Respiration of aquatic sediments often comprises over 50% of the total respiration of aquatic systems, and plays a tremendous role in the global carbon cycle. The present thesis deals with the impacts of the physical environment (sediment characteristics, mainly hydraulic conductivity and grain fractions) on the (microbial) respiration of bioturbated sediments. In order to disentangle the effects of bioturbation on respiration, a novel measurement method has been developed (Chapter 4.1). Chapter 4.2 reveals that the impact of bioturbation on sediment respiration increases with increasing temperature. Chapter 4.3 shows that resazurin can also be used for the measurement of respiration in bioirrigated marine sediments. Chapter 4.4 reviews the large number and diversity of hydrological, biogeochemical and ecological tracers including resazurin. The present thesis shows that in sediments with low hydraulic conductivity (diffusion-dominated sediments) (Chapters 4.1,4.2) bioturbation is altering sediment respiration to a larger extent than in sediments with high hydraulic conductivity (advection-dominated sediment) (Chapter 4.3). The physical environment (sediment matrix) controls the intensity of the impacts of bioturbation on sediment respiration. Thus, this thesis provides a basis for understanding the impact of benthic bioturbators on respiration and carbon sequestering in freshwater and marine sediments.

The aquatic environment and zoobenthos are closely related in a water body. In recent years, the use of zoobenthos to evaluate water quality is getting more attention as this approach is less polluting and less costly. This study was conducted to assess the diversity of zoobenthos in the canals affected by leachate and agricultural activities. Five sediment samples were collected in two campaigns, one in April and one in October 2018. Water samples were also collected at the same time for zoobenthos to assess the water quality and serve as a reference for assessing the effectiveness of using zoobenthos as water quality indication. In total, 17 species of zoobenthos belonging to six families and four classes were identified, of which Limnodrilus hoffmeisteri and Tendipes species being present regularly at sampling sites through the surveys. The calculations of the Shannon-Wiener diversity index (H'), rapid bioassessment protocol (RBP III), and associated average score per taxon (ASPT) indices based on species composition indicated that the water environment surrounding the landfill was moderately to severely polluted. However, the water quality index (WQI) calculated based on the physical and chemical properties shows that the level of water pollution in canals was less than that evaluated using zoobenthos. This can be explained by the fact that zoobenthos also affected the properties of sediments which depend on the water column. The findings in this study showed that the aquatic environment around the landfill is heavily contaminated as result of waste disposal and agricultural activities. The use of both zoobenthos combined with physical and chemical indicators could be useful in assessing the canals’ water status. / Môi trường nước và động vật đáy có liên quan mật thiết với nhau trong một thủy vực. Trong những năm gần đây, việc sử dụng động vật đáy để đánh giá chất lượng môi trường nước mặt được quan tâm vì phương pháp này ít gây ô nhiễm môi trường và ít tốn kém chi phí. Nghiên cứu được tiến hành để đánh giá sự phân bố của động vật đáy trong hệ thống kênh rạch chịu tác động từ nước rỉ rác và các hoạt động sản xuất nông nghiệp. Năm mẫu động vật đáy được thu hai đợt, đợt 1 vào tháng 4 và đợt 2 vào tháng 10 năm 2018. Mẫu nước cũng được thu để đánh giá chất lượng nước và làm cơ sở đánh giá hiệu quả của việc sử dụng động vật đáy làm chỉ thị chất lượng môi trường nước. Kết quả nghiên cứu cho thấy thành phần động vật đáy phát hiện 17 loài thuộc 6 họ và 4 lớp, trong đó các loài Limnodrilus hoffmeisteri và loài Tendipes hiện diện thường xuyên ở tất cả các điểm thu mẫu qua hai đợt khảo sát. Dựa vào thành phần loài tính toán các chỉ số Shannon-Wiener (H’), chỉ số đánh giá nhanh sinh học (RBP III), và chỉ số tính điểm trung bình bình theo họ (ASPT) cho thấy môi trường ô nhiễm rất nặng trong khi đó chỉ số WQI được tính toán dựa vào các chỉ tiêu lý hóa cho thấy mức độ ô nhiễm nước ở kênh xung quanh bãi rác chỉ ở mức nhẹ hơn. Như vậy, việc sử dụng động vật đáy cho kết quả đánh giá chất lượng nước với mức độ ô nhiễm cao hơn. Điều này có thể giải thích là do động vật đáy chịu ảnh hưởng bởi đặc tính nền đáy và cột nước bên trên nền đáy. Kết quả nghiên cứu cho thấy môi trường nước xung quanh bãi rác bị ô nhiễm nặng do xả thải và hoạt động sản xuất nông nghiệp. Việc sử dụng động vật đáy kết hợp với các chỉ tiêu lý, hóa có thể hữu ích hơn trong việc đánh giá hiện trạng nước kênh.

The study aimed to evaluate water quality at Bung Binh Thien Lake, An Giang Province, Vietnam using Shannon-Wiener species diversity index (H’) and associated average score per taxon (ASPT) calculated from composition of phytoplankton and zoobenthos. The water quality index (WQI) was used as the reference for the quality of surface water. The samples of surface water quality, phytoplankton, and zoobenthos were simultaneously collected at 11 sites in the dry season. The results showed that WQI (57-88) classified water quality from good to medium, H’ calculated using phytoplankton species (1.12-2.71) presented water quality from medium to bad whereas, (H'z) calculated (0 to 2.07) and ASPT (2-4.21) calculated from zoobenthos species divided water quality from bad to very bad. The findings revealed that assessing water quality should not totally only relied on diversity indices (H’, ASPT) but also carefully consider compositions of phytoplankton and zooplankton. In addition, interpretation of the biodiversity indices for water quality examination should involve the experts in the relevant fields. / Nghiên cứu nhằm đánh giá chất lượng nước tại hồ Bung Binh Thiên, tỉnh An Giang, Việt Nam sử dụng chỉ số đa dạng loài Shannon-Wiener (H’) và chỉ số tính điểm trung bình bình theo họ (ASPT) được tính từ thành phần của phiêu sinh thực vật và động vật đáy. Chỉ số chất lượng nước (WQI) được sử dụng tham chiếu cho chất lượng nước mặt. Các mẫu chất lượng nước mặt, phiêu sinh thực vật thực vật và động vật đáy được thu đồng thời tại 11 địa điểm trong mùa khô. Kết quả cho thấy WQI (57-88) phân loại chất lượng nước từ tốt đến trung bình, H’p được tính dựa vào các loài phiêu sinh thực vật (1.12-2.71) thể hiện chất lượng nước từ trung bình đến xấu trong khi, H’z (0- 2.07 ) và ASPT (2-4,21) được tính toán từ các loài động vật đáy phân loại chất lượng nước từ xấu đến rất xấu Kết quả nghiên cứu cho thấy đánh giá chất lượng nước không chỉ hoàn toàn dựa vào các chỉ số đa dạng (H’, ASPT) mà còn xem xét cẩn thận thành phần loài của phiêu sinh thực vật và động vật đáy. Ngoài ra, việc giải thích các chỉ số đa dạng sinh học nhằm tra chất lượng nước cần có sự tham gia của các chuyên gia trong các lĩnh vực liên quan.

The impact of vertical distribution of zoobenthos in a pond in response to the biomass stocking of carp ponds was tested in the Rod, Dobrá Vůle, Pohořelec, Haslauerteich and Neuteich ponds. Except for the Rod pond, various carp biomass was represented in all ponds. Sampling was conducted during the growing season (June and July), and outside the growing season (September). Physico-chemical parameters of water were measured for each sample. All samples were sorted by groups, counted and weighed. The results are presented in graphs. Zoobenthos density of the Dobrá Vůle, Haslauerteich and Neuteich ponds showed a rising tendency towards deeper parts of the pond sediment. The density of these ponds reached the maximum values in depths of 10-14 cm. This tendency was observed in the occurrence of midges in these ponds. The highest density of midges was observed in the Haslauerteich pond (1,687 individuals per square metre). The above-mentioned ponds were stocked by the common carp (K2-3). The largest density of the tubifexes in the Dobrá Vůle, Haslauerteich and Neuteich ponds was observed during the September sampling in the shallow parts of the sediment (0-6 cm). The maximum density was 1,031 individuals per square metre. On the Pohořelec pond, the occurrence of zoobenthos during both samplings was recorded only in the depth of 10-12 cm. Other representatives of zoobenthos were present in the Rod, Haslauerteich and Neuteich ponds. Larvae of Sialis and Chaoboridae were observed as well. The density and biomass was greater in the September sampling. We have noticed a greater settlement in various depths. Their maximum density was in the Haslauerteich pond (218 individuals per square metre).

Devido à importância do uso de bioindicadores para avaliação da qualidade ambiental e às vantagens do emprego de macroinvertebrados bentônicos para este fim, estes organismos foram utilizados para avaliar a qualidade da água da bacia do alto Atibaia (SP) em oito pontos de amostragem: 3 no rio Atibainha, 3 no rio Cachoeira e 2 no rio Atibaia. Foram amostrados o canal, a margem deposicional e a margem erosional, sendo coletadas três unidades amostrais em cada ambiente de cada ponto, com auxílio do amostrador Hess (rede de 500 μm e área de 678,9 cm2) ou pegador Petersen modificado (área de pegada de 325 cm2). Foram coletadas amostras de sedimento com pegador Petersen modificado em aço inox para análise de granulometria, resíduos totais, voláteis e fixos e umidade. Este trabalho está dividido em três capítulos. No primeiro, a análise da fauna evidenciou que os pontos de cabeceira e à jusante dos reservatórios dos rios Atibainha e Cachoeira tenderam a apresentar riqueza e diversidade menores que os pontos de localização intermediária. Já nos pontos situados no rio Atibaia, a fauna respondeu à deterioração da qualidade da água, refletindo um gradiente ambiental. No segundo capítulo, visando à utilização de técnicas de avaliação rápida para facilitar a análise e interpretação de dados, foram aplicadas vinte e uma métricas biológicas. Dentre todas as métricas testadas, as que evidenciaram o gradiente ambiental presente na bacia estudada, com restrito coeficiente de variação no ponto referência foram riqueza e diversidade de famílias, densidade total, riqueza de EPT e ETO, porcentagem de Táxons de Diptera (exceto Tanytarsini) e não insetos, BMWP “JUNQ” e IBBJ. No terceiro capítulo, com a finalidade de contribuir para a elaboração de um plano de monitoramento de avaliação rápida para a bacia do Alto Atibaia (SP), concluiu-se que, com a metodologia empregada, a margem erosional seria o habitat que melhor corresponderia à media ou somatória para o ponto, pelo menos durante o período de estiagem. / Due to importance of the use of bioindicators to assess environmental quality and the advantages of employing benthic macroinvertebrates for this purpose, these organisms were used to assess the water quality of the Atibaia river basin (SP) in eight sites: 3 in the Atibainha river, 3 in the Cachoeira river, and 2 in the Atibaia river. Channel, depositional margin and erosional margin were sampled. Three replicates were collected in each habitat using a Hess sampler (678.9 cm2, 500μm mesh) or modified Petersen grab (325 cm2). Sediment samples were collected with a stainless steel modified Petersen grab for analysis of granulometry, humidity and total, volatile and fixed residues. This study is divided in three chapters. In the first, the fauna showed that headwater and downstream reservoir sites of Cachoeira and Atibainha rivers presented lower richness and diversity than intermediate sites. In the sampling sites of the Atibaia river, the fauna responded to the water quality decline, reflecting an environmental gradient. In the second chapter, aiming the utilization of rapid assessment techniques to facility data analysis and interpretation, twenty one biological metrics were applied. Metrics that evidence the environmental gradient of the basin, with restricted variation coefficient, were richness and diversity of families, total density, richness of EPT and ETO, percent of Diptera taxa and noninsects, BMWP “JUNQ” and IBBJ. In the third chapter, aiming to contribute to the development of a rapid assessment monitoring plan for the Atibaia river basin, according to the methodology employed, it was concluded that the erosional margin was the habitat that showed the best correspondence with the average or sum of the sites, at least during the dry season.

The impact of vertical distribution of zoobenthos in a pond in response to the biomass stocking of carp ponds was tested in the Rod, Dobrá Vůle, Pohořelec, Haslauerteich and Neuteich ponds. Except for the Rod pond, various carp biomass was represented in all ponds. Sampling was conducted during the growing season (June and July), and outside the growing season (September). Physico-chemical parameters of water were measured for each sample. All samples were sorted by groups, counted and weighed. The results are presented in graphs. Zoobenthos density of the Dobrá Vůle, Haslauerteich and Neuteich ponds showed a rising tendency towards deeper parts of the pond sediment. The density of these ponds reached the maximum values in depths of 10-14 cm. This tendency was observed in the occurrence of midges in these ponds. The highest density of midges was observed in the Haslauerteich pond (1,687 individuals per square metre). The above-mentioned ponds were stocked by the common carp (K2-3). The largest density of the tubifexes in the Dobrá Vůle, Haslauerteich and Neuteich ponds was observed during the September sampling in the shallow parts of the sediment (0-6 cm). The maximum density was 1,031 individuals per square metre. On the Pohořelec pond, the occurrence of zoobenthos during both samplings was recorded only in the depth of 10-12 cm. Other representatives of zoobenthos were present in the Rod, Haslauerteich and Neuteich ponds. Larvae of Sialis and Chaoboridae were observed as well. The density and biomass was greater in the September sampling. We have noticed a greater settlement in various depths. Their maximum density was in the Haslauerteich pond (218 individuals per square metre).

Devido à importância do uso de bioindicadores para avaliação da qualidade ambiental e às vantagens do emprego de macroinvertebrados bentônicos para este fim, estes organismos foram utilizados para avaliar a qualidade da água da bacia do alto Atibaia (SP) em oito pontos de amostragem: 3 no rio Atibainha, 3 no rio Cachoeira e 2 no rio Atibaia. Foram amostrados o canal, a margem deposicional e a margem erosional, sendo coletadas três unidades amostrais em cada ambiente de cada ponto, com auxílio do amostrador Hess (rede de 500 μm e área de 678,9 cm2) ou pegador Petersen modificado (área de pegada de 325 cm2). Foram coletadas amostras de sedimento com pegador Petersen modificado em aço inox para análise de granulometria, resíduos totais, voláteis e fixos e umidade. Este trabalho está dividido em três capítulos. No primeiro, a análise da fauna evidenciou que os pontos de cabeceira e à jusante dos reservatórios dos rios Atibainha e Cachoeira tenderam a apresentar riqueza e diversidade menores que os pontos de localização intermediária. Já nos pontos situados no rio Atibaia, a fauna respondeu à deterioração da qualidade da água, refletindo um gradiente ambiental. No segundo capítulo, visando à utilização de técnicas de avaliação rápida para facilitar a análise e interpretação de dados, foram aplicadas vinte e uma métricas biológicas. Dentre todas as métricas testadas, as que evidenciaram o gradiente ambiental presente na bacia estudada, com restrito coeficiente de variação no ponto referência foram riqueza e diversidade de famílias, densidade total, riqueza de EPT e ETO, porcentagem de Táxons de Diptera (exceto Tanytarsini) e não insetos, BMWP “JUNQ" e IBBJ. No terceiro capítulo, com a finalidade de contribuir para a elaboração de um plano de monitoramento de avaliação rápida para a bacia do Alto Atibaia (SP), concluiu-se que, com a metodologia empregada, a margem erosional seria o habitat que melhor corresponderia à media ou somatória para o ponto, pelo menos durante o período de estiagem. / Due to importance of the use of bioindicators to assess environmental quality and the advantages of employing benthic macroinvertebrates for this purpose, these organisms were used to assess the water quality of the Atibaia river basin (SP) in eight sites: 3 in the Atibainha river, 3 in the Cachoeira river, and 2 in the Atibaia river. Channel, depositional margin and erosional margin were sampled. Three replicates were collected in each habitat using a Hess sampler (678.9 cm2, 500μm mesh) or modified Petersen grab (325 cm2). Sediment samples were collected with a stainless steel modified Petersen grab for analysis of granulometry, humidity and total, volatile and fixed residues. This study is divided in three chapters. In the first, the fauna showed that headwater and downstream reservoir sites of Cachoeira and Atibainha rivers presented lower richness and diversity than intermediate sites. In the sampling sites of the Atibaia river, the fauna responded to the water quality decline, reflecting an environmental gradient. In the second chapter, aiming the utilization of rapid assessment techniques to facility data analysis and interpretation, twenty one biological metrics were applied. Metrics that evidence the environmental gradient of the basin, with restricted variation coefficient, were richness and diversity of families, total density, richness of EPT and ETO, percent of Diptera taxa and noninsects, BMWP “JUNQ" and IBBJ. In the third chapter, aiming to contribute to the development of a rapid assessment monitoring plan for the Atibaia river basin, according to the methodology employed, it was concluded that the erosional margin was the habitat that showed the best correspondence with the average or sum of the sites, at least during the dry season.

Les plages de sable sont des écosystèmes dynamiques couvrant 70% des côtes mondiales. Ces systèmes abritent un cortège spécifique unique et assurent des fonctions essentielles de nourricerie, de nurserie et d’épuration des eaux. Or à proximité des côtes, la population humaine est en développement constant ce qui accroît les multiples pressions d’origine anthropique sur les écosystèmes côtiers. L’apport en excès de nutriments constitue une menace majeure qui peut se traduire par le développement d’importantes biomasses de macroalgues opportunistes (eutrophisation). La fréquence et l’intensité de ces blooms, communément formés de chlorophycées à court cycle de vie (ulves) et appelés marées vertes (MV), s'amplifient sur les côtes françaises et dans le Monde menaçant le fonctionnement de systèmes prépondérants et uniques.La plupart des études visant à déterminer les effets des MV sur la structure et le fonctionnement d’écosystèmes sédimentaires ont été conduites dans des environnements abrités, micro- ou atidaux. Cette problématique est restée presqu’inexplorée dans des écosystèmes plus dynamiques (systèmes ouverts et macrotidaux) en raison des difficultés de mise en place d’échantillonnage et de détection des effets de stress d’origine anthropique, inhérentes à la variabilité de ces systèmes. Cette thèse a donc eu pour objectif principal de combler ce manque et produire des connaissances en étudiant les réponses in situ des communautés benthiques de plages de sable fin macrotidales en présence ou non de MV. Quatre études ont été menées à différentes échelles spatio-temporelles et en considérant différents compartiments biologiques pour répondre à cette question générale.Ce travail de thèse montre qu’à l’échelle de la région Bretagne (variabilité intégrée sur 2700km de côtes et 7 ans) les communautés benthiques d’écosystèmes dynamiques sont modifiées significativement et de manière conservative par la présence de MV. Ce travail démontre aussi que les marées vertes impactent différemment la faune benthique en fonction du type d’habitat (plages semi-exposées vs. exposées), de la profondeur (mediolittoral vs. infralittoral), et du compartiment biologique (macrofaune benthique vs. juvéniles de poissons plats). Ces comparaisons ont permis d’identifier la faune benthique de médiolittoral inférieur des plages exposées comme étant le système le plus affecté par les MV. L’étude des variations à fine échelle spatio-temporelle de ce dernier montre que les caractéristiques faunistiques (uni- et multi-variées) sont modifiées le long d’un gradient de couverture d’algues vertes. Par exemple, la β-diversité décroît significativement le long de ce gradient. Afin d’explorer les processus pouvant expliquer ces modifications, et déterminer si ces changements ont des répercussions sur le fonctionnement de l’écosystème « plage de sable », les effets de l’accumulation d’ulves sur le réseau trophique à différent(e)s niveaux/échelles ont été mesurés. Les résultats de cette étude montrent qu’une importante biomasse d’ulves induit un changement de la structure entière du réseau trophique et une modification importante du fonctionnement trophique des plages. Les expérimentations menées au cours de cette thèse montrent que les changements observés sont induits par des effets directs (consommation de débris d’ulves) et indirects (modifications d’autres sources de nourriture) de la présence des MV.Cette thèse propose un cadre de travail visant à mieux détecter les effets de stress anthropiques sur la structure et le fonctionnement d’écosystèmes dynamiques. Dans un contexte de changement global forçant les écosystèmes à faire face à de multiples stress, cette approche pourrait se révéler particulièrement utile pour démêler, comprendre et prédire les effets de perturbations induites par les activités humaines sur le fonctionnement des écosystèmes et constituer une aide à la gestion de ces environnements particuliers. / Highly dynamic systems, often considered as resilient systems, are characterised by abiotic and biotic processes under continuous and strong changes in space and time. Because of this variability, the detection of overlapping anthropogenic stress is challenging. Coastal areas harbour dynamic ecosystems in the for of open sandy beaches, which cover the vast majority of the world’s ice-free coastline. These ecosystems are currently threatened by increasing human-induced pressure, among which mass-development of opportunistic macroalgae (mainly composed of Chlorophyta, so called green tides), resulting from the eutrophication of coastal waters. The ecological impact of opportunistic macroalgal blooms (green tides, and blooms formed by other opportunistic taxa), has long been evaluated within sheltered and non-tidal ecosystems. Little is known, however, on how more dynamic ecosystems, such as open macrotidal sandy beaches, respond to such stress. This thesis assesses the effects of anthropogenic stress on the structure and the functioning of highly dynamic ecosystems using sandy beaches impacted by green tides as a study case. The thesis is based on four field studies, which analyse natural sandy sediment benthic community dynamics over several temporal (from month to multi-year) and spatial (from local to regional) scales. In this thesis, I report long-lasting responses of sandy beach benthic invertebrate communities to green tides, across thousands of kilometres and over seven years; and highlight more pronounced responses of zoobenthos living in exposed sandy beaches compared to semi-exposed sands. Within exposed sandy sediments, and across a vertical scale (from inshore to nearshore sandy habitats), I also demonstrate that the effects of the presence of algal mats on intertidal benthic invertebrate communities is more pronounced than that on subtidal benthic invertebrate assemblages, but also than on flatfish communities. Focussing on small-scale variations in the most affected faunal group (i.e. benthic invertebrates living at low shore), this thesis reveals a decrease in overall beta-diversity along a eutrophication-gradient manifested in the form of green tides, as well as the increasing importance of biological variables in explaining ecological variability of sandy beach macrobenthic assemblages along the same gradient. To illustrate the processes associated with the structural shifts observed where green tides occurred, I investigated the effects of high biomasses of opportunistic macroalgae (Ulva spp.) on the trophic structure and functioning of sandy beaches. This work reveals a progressive simplification of sandy beach food web structure and a modification of energy pathways over time, through direct and indirect effects of Ulva mats on several trophic levels. Through this thesis I demonstrate that highly dynamic systems respond differently (e.g. shift in δ13C, not in δ15N) and more subtly (e.g. no mass-mortality in benthos was found) to anthropogenic stress compared to what has been previously shown within more sheltered and non-tidal systems. Obtaining these results would not have been possible without the approach used through this work; I thus present a framework coupling field investigations with analytical approaches to describe shifts in highly variable ecosystems under human-induced stress.